Every application should have a user table for in the database. To my knowledge, OpenSSO JDBC need 5 mandatory columns: User_Name, Password, Active_Indicator, sun-fm-saml2-nameid-info, sun-fm-saml2-nameid-infokey. So if you have the user table that already has the User_Name, Password, Active_Indicator, then you just need to add sun-fm-saml2-nameid-info, sun-fm-saml2-nameid-infokey columns to your existing table. Or you can create a new table with these columns and have database trigger (insert/update/delete) to this table.
Please note, you can change those column names to any name that you want. We can do the mapping in the configuration later on.