I think the best way to approach this is to implement custom Membership provider (on top of MonoSoftware.MonoX.MonoXMembershipProvider
which inherits AspNetSqlMembershipProvider
) and custom Role manager (on top MonoSoftware.MonoX.MonoXRoleProvider
which inherits AspNetSqlRoleProvider
). That way you can use MonoX database for the CMS and your own database for the user management.
If you choose this approach let us know so we can provide you with the additional information that you will need to perform this task.
Some references that may help
Introduction to Membership
You can read more about extending ASP.NET membership here