<PAGE>   1
                                                                   Exhibit 10.21

                SUPPLEMENTAL LIMITED LIABILITY COMPANY AGREEMENT



                  This SUPPLEMENTAL LIMITED LIABILITY COMPANY AGREEMENT, dated
as of March     , 2000 (the "Agreement"), is entered into by and between WBT
Holdings LLC, a Delaware limited liability company (the "Company"), and the
individual listed on the signature page hereto (the "Individual").

                  WHEREAS, the Company desires to sell Units to
an additional member of the Company's management;

                  WHEREAS, the Individual desires to subscribe for and purchase,
and the Company desires to sell to the Individual the number of Units (the
"Units") subscribed for by the Individual pursuant to a Subscription Agreement,
dated as of the date hereof (the "Subscription Agreement"), between the Company
and the Individual; and

                  WHEREAS, the Company and the Individual desire to enter into
this Agreement in order to set forth their understanding with respect to certain
voting arrangements, transfer rights and other matters relating to the
Individual's ownership of Units and to acknowledge that any Units acquired by
the Individual pursuant to the Subscription Agreement are subject to, and
governed by, the Limited Liability Company Agreement (as defined below).

                  NOW, THEREFORE, in consideration of the mutual promises
contained in this Agreement and other good and valuable consideration the
receipt and sufficiency of which is hereby acknowledged, the parties to this
Agreement agree as follows:

                                    ARTICLE I

              INCORPORATION OF LIMITED LIABILITY COMPANY AGREEMENT

                  SECTION 1.1 Documentation With Respect to the Shares. The
Individual hereby acknowledges that the Units are subject to various agreements,
including with out limitation, the Limited Liability Company Agreement, dated as
of February 10, 2000 (as the same may be amended from time to time, the "Limited
Liability Company Agree-



                                       1
<PAGE>   2
ment"), by and among the Company and the Members of the Company as set forth
therein. Terms not defined herein shall have the meanings set forth in the
Limited Liability Company Agreement.

                  SECTION 1.2 Access to Information. The Individual (i) has
reviewed the Limited Liability Company Agreement and the other materials
furnished to him in connection with the transactions contemplated hereby and
contemplated by the Subscription Agreement and (ii) has been granted the
opportunity to ask questions of, and has received answers from, representatives
of the Company and its affiliates concerning the terms and conditions of the
purchase of the Units and has been granted the opportunity to obtain any
additional information that he or she deems necessary to verify the accuracy of
the information contained in the Limited Liability Company Agreement and the
Subscription Agreement and such other materials.


                  SECTION 1.3 General. Upon the execution of this Agreement, the
Individual shall be deemed, in accordance with Article XIV of the Limited
Liability Company Agreement, to have the same rights and obligations as a Member
(as defined therein) for purposes of the Limited Liability Company Agreement.
The Individual hereby agrees to be bound by, and agrees that any Units acquired
pursuant to the Subscription Agreement are subject to, and governed by, the
terms and conditions of the Limited Liability Company Agreement. The Individual,
in accordance with Article XIV of the Limited Liability Company Agreement, is
hereby deemed to be a Management Member (as defined therein) for all purposes of
the Limited Liability Company Agreement. Pursuant to Section 14.3 of the
Limited Liability Company Agreement, this Agreement is effective as of the date
hereof.

                  SECTION 1.4 Conflict. In the event of a conflict between the
terms hereof and the Limited Liability Company Agreement, the terms of the
Limited Liability Company Agreement shall control.




                                       2
<PAGE>   3
                                   ARTICLE II

                                  Miscellaneous

                  SECTION 2.1 After-Acquired Units. All of the provisions of
this Agreement shall apply to all of the securities of the Company now owned or
which may be issued or Transferred (as defined in the Limited Liability Company
Agreement) hereafter to the Individual in consequence of any additional
issuance, purchase (including, without limitation, pursuant to options to
purchase capital stock of the Company), exchange, conversion, or
reclassification of securities, corporate reorganization, or any other form of
recapitalization, consolidation, merger, unit split or unit dividend, or which
are acquired by the Individual in any other manner (all of which shall be
deemed to be "Units" hereunder).

                  SECTION 2.2 Specific Performance. The parties hereto
acknowledge that irreparable damage would result if this Agreement is not
specifically enforced and that, therefore, the rights and obligations of the
parties under this Agreement may be enforced by a decree of specific performance
issued by a court of competent jurisdiction, and appropriate injunctive relief
may be applied for and granted in connection therewith. Such remedies shall,
however, be cumulative and not exclusive and shall be in addition to any other
remedies which any party may have under this Agreement or otherwise.

                  SECTION 2.3 Terms of Agreement. This Agreement shall become
effective on the date hereof and shall continue in full force and effect until
the date on which the Limited Liability Company Agreement terminates.

                  SECTION 2.4 Successors and Assigns. All the terms and
provisions of this Agreement shall inure to the benefit of and be binding upon
the parties hereto and their respective successors and assigns; provided, how
ever, no party hereto may assign this Agreement or its rights or obligations
hereunder without the prior written consent of the other party hereto.

                  SECTION 2.5 Entire Agreement. This Agreement, the Limited
Liability Company Agreement and the Subscription Agreement constitute the
complete understanding and agreement among the parties hereto with respect to
the



                                       3
<PAGE>   4
subject matter hereof and this Agreement can be amended, supplemented or
changed, and any provision hereof can be waived, only by written instrument
making specific reference to this Agreement signed by the party against whom
enforcement of any such amendment, supplement, modification or waiver is
sought.

                  SECTION 2.6 Approvals and Consents. The Individual hereby
agrees, for himself or herself, his or her heirs and legal representatives(s),
to prepare, execute and deliver or cause to be prepared, executed and delivered
such further instruments and documents, and to take such other actions as may be
reasonably required to more effectively carry out the intent and purposes of
this Agreement and the Limited Liability Company Agreement and the transactions
contemplated hereby and thereby.

                  SECTION 2.7 Notices. All notices, requests, demands and other
communications which are required or may be given hereunder shall be in writing
and shall be deemed to have been duly given to a party if delivered personally
or mailed by certified or registered mail, return receipt requested, postage
prepaid, or if given by telex or other telegraphic means, to such party at his
or its address set forth in Schedule A hereto. Any such notice shall for all
purposes of this Agreement be deemed to have been given, and received by the
addressee, on the date of such personal delivery or telex or telegraphic notice
or five (5) days after any such mailing thereof.

                  SECTION 2.8 Governing Law. This Agreement shall be governed by
and construed in accordance with the laws of the State of Delaware.

                  SECTION 2.9 Severability. If any provision of this Agreement
or the application of any such provision to any person or circumstances shall
be held invalid by a court of competent jurisdiction, the remainder of this
Agreement, including the remainder of the provision held invalid, or the
application of such provision to persons or circumstances other than those as to
which it is held invalid, shall not be affected thereby.

                  SECTION 2.10  Counterparts.  This Agreement may be executed in
one or more counterparts, each of which



                                       4
<PAGE>   5
shall be deemed to be an original, but all of which together shall constitute
one and the same instrument.

                  SECTION 2.11 Headings. All section headings herein are for
convenience of reference and are not part of this Agreement, and no construction
or interference shall be derived therefrom.



                                       5
<PAGE>   6
                  IN WITNESS WHEREOF, the parties hereto have hereunto set their
hands as of the date first set forth above.


                                                     ___________________________
                                                     Name: Lance D'Amico


                                                     WBT HOLDINGS LLC



                                                     By:________________________
                                                        Name:  Robert Fogelson
                                                               Title:



                                       6
<PAGE>   7
                                                                      Schedule A

Individual:

Lance D'Amico






Company:

WBT HOLDINGS LLC






Units Purchased:           5,000

Capital Account:


                                       7