Hmmm....
So you have a list of users and roles in separate tables that don't seem to be linked together, but you do have the user_roles table that seems to have it.
First you would need to set up a repository and let it know that all of your roles come from T_ROLES. (this might need to be a custom piece modeled after one of the initial load jobs) It would also set up the ONLY and SYSTEM roles.
So it would seem that when you assign the ONLY role it would trigger a task that creates the user in T_USERS.
Then you would be able to assign the previously loaded repository roles and assign them to users with the application assigned to them.
If you need I can model this more on my test system. This is all coming out of my head at the moment.
Matt