You may find this useful:
Scott shows you how to customize the CreateUserWizard to insert extra Profile data. Instead of that, you can also redirect to another page where you store data in the Profile from fields on the form directly.
Instead of using the Profile, you can do the same with your own table. You can use the user's UserName and Application name of the Membership provider as the primary key for your table / user. You could also use the user's ProviderUserKey (a Guid) as well, although Microsoft recommends not to use it as it may be changed in the future.
Does this help?