You'd have to add the additional fields to the MembershipReviewList. Then you'd have to modify MembershipRequest.cs to add the additional fields when records get inserted into the MembershipReviewList.
Also, if you'd like to display the values in the User Management screen, you'd need to modify FBAUsersView.cs to return the extra columns, and then you'd have to modify the UserXXXX.aspx pages to reference the new fields.
BTW, where are you storing this additional data? In the membership database? If you're going to be sharing it with other applications, that's probably the place to put it. If you're only going to only be using it in SharePoint, you might want
to consider storing it in SharePoint's User Information List. Then it's easily queried within SharePoint along with the rest of the user's profile information. As well it will be visible on the user information list pages.