The "Multiplayer-ID" is a code that is used to identify your account on the server. That means if loose your ID, you will also loose the access to all our characters on server.
Although its an ID used to identify you server side, the server cannot recover that code again. Administrators are not able to retrieve the IDs of the players. Once you lost your ID code, your account is lost too. Therefore, we just can encourage you to store your ID separated and not on your PC (at old times people used to print something out to have a copy elsewhere).
You can find your own ID by having a look at the following file within your Freelancer\\EXE folder: myflaccountid.hta
You can also create and use more than one IDs and and so multi player accounts. This applies to all mods and not solely Crossfire.
Folling link provides you some easy steps to follow, in order to have multiple accouts available: Tutorial for CREATING additional Server Accounts. The tool used and described is called Freelancer Account Manager NG.
Before you create a new ID make sure that you have backed up your old one, otherwise this will be lost too.