the .net 20 membership provider needs to run on its own database schema. You won't be able to use your login control to authenticate against your own custom schema without some major hack.
however, it shouldn't be too hard to integrate membership provider schema into your existing db.
1)you'll need to run aspnet_regsql.exe to setup the schema into your db
2)you'll need to do a little T-SQL to map your fields into (or use .net pro grammatically done if password is going to be hashed) ASPNET_(whatever)table
3)Maybe (although most likely not) you'll need to fill out some bogus data because membership provider schema may have fields you never had.
4)modify your web.config and you are ready to use .net20 membership provider controls