Can a legal document be made similar to a prenuptial agreement after the marriage has occurred if both parties agree?

The two of you can execute a Post Marital Agreement (similar to a prenup but executed after you are married). The main difference between a prenup and a postnup is there has to be consideration given for a postnup when no consideration is required for a prenup. As such, it is important you have an attorney draft your Post Marital Agreement to ensure it is as enforceable as possible.

I agree. You can certainly have a postnuptial agreement prepared. The critical question is whether or not the other party will sign it though. You could go to the expense of preparing the agreement but you cannot force your spouse to sign it. With a premarital agreement, chances are the marriage will not take place unless both parties sign the agreement. You don't have that situation when you're already married.