Multiple Authentication Methods?

I'm wondering if it is possible (either internally or via a plugin someone has made) to use multiple authentication methods.
Basically, I would like to have internal and pam both in use, so that users I create on my server automatically have an account, but users can also create accounts directly. Obviously such a case would need to verify that internal users do not conflict with system users since you couldn't have 2 users with the same username otherwise interaction with them would probably get messy very quickly.
I didn't see anything mentioning whether this is possible, so I figured I would ask.

Thanks.

*Edited a couple times*