I agree that to "do it right" you should use an external mixer.
It might be possible to make this work with Soundflower and an app that will give you control over the mic mix/gain. I'm not coming up with anything useful suggestions for an app, off the top of my head. But, a quick search at macupdate, and I found this:http://www.ardour.org/
...which should be able to handle the routing on it's own (sans Soundflower.)
Not sure this would play nicely with RLDJ, but then there's no reason to suspect that it wont. The features suggest the ability to route anything to anything, and mixing.
If the USB connection presents itself as a choice on the input selections in System Preferences, you might be able to make it your default input. You could then use the setting in RLDJ to make 'Line In' your system input... It would be possible to cut the mic on and off, roughly... And gain control (if present at all) would be in the Audio portion of the System Prefs. Not the best, but it might 'work'.