i'm not on my mac right now, but i know you can daisy chain inputs using the Aggregate Device Editor found in the Audio/MIDI Setup (in Utilities). i'm pretty sure the same would work for output, but i haven't tried it myself.
you can for example, link the built-in mic as well as the line in.
I'm going to check this out. If you have any more information on doing it this way, I'd greatly appreciate it.
By the way, I changed my user ID to this one. I'm the OP. Thanks!