Mixed authentication Forms and Windows

Hi all,
 
I am working on a web application where the customers can be both internal and external which requires me to use mixed authentication. The internal users also have multiple roles like manager, admin etc..
The Orders table should have a foreign key to the Users table and my problem is
1) How do I handle the Users table? I do not wish to put both internal and external users in one table.
I am thinking about using sqlmembership with sqlroleprovider but I do not know how to proceed. I will also need to add additional columns to the aspnet_users table.
Any direction or help will be appreciated.
Thankyou.
Cheedep.
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