This is a challenging problem -- I don't think there is an easy solution.
The quickest way I can think of is to simply combine login functionality but leave the rest of the functionality separate. You might want to re-skin your app's admin interface to look like c5's, so it doesn't seem like separate systems to the user (or just keep them separate and explain to users that that's just the way it is).

How to combine logins? Well, assuming both sites are on the same domain, you can share the session cookie between the two systems. You will want to have a look into the C5 core code to see how it reads and saves this session data, and then modify your app to work that way as well (it will be infinitely easier to modify your own code than to try and change C5's code). You may need to rig up some system where upon first login to the C5 site, if no user exists, it runs a query against your other system to see if the user exists there, and if so import that one user's data over to the C5 system (basically register them behind the scenes using data from the other system). You'll run into problems in terms of synching future profile updates, though, so you need to decide if you care about that or not (if you do care, you will need to write more synching code that is triggered on a profile update from either system).

There are definitely other ways to do it as well, but off the top of my head that seems like the one I would try.

-Jordan

Thank you, this is how I was thinking about it too. I'll try to stay out of C5's way as much as possible with the login part.

Has someone tried to use their own database for logging in to C5 already? Any tips / gotchas?