The prenuptual agreement is a contract--nothing more, and nothing less. Like all contracts, it can be modified at will by the all the parties to it. That is, while neither you nor your husband can unilaterally (on your own) change the terms of the agreement, together you may change the terms however you like, including by adding an addendum: simply draft it and sign it. Of course, you should consult with an attorney before doing this, because you want (1) to make sure that with the change you are making, the agreement is still accomplishing what you want it to (i.e. you may want to change some other term to account for this change); and (2) to make sure that you take cognizance of and plan for any tax implications or consequences. Given how much money is at stake, hiring an attorney to help you do this is a good investment.