How to use role management without membership?

I would like to take advantage of sql-based role management, but would like to use my own existing tables and code for users (membership). 

I read in the online docs this is possible, but no clues of how to do this.  When I look at the default aspnet app services db, the UserRoles table has a foreign key on the UserId to the asp_Users table.  I would like for that UserId column to be referring to my own user table.

Is there some sort of easy config setting I can make to make this happen?  Or do I need to manually drop that foreign key?  (And won't it still look for users there?)  Is there a way to install the app services db WITHOUT the membership tables? 

thanks in advance,

-Stacy
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