Yes, all you do is put a compressor on the track as an insert, and voila.
You'll have to make sure you're listening to the A/D/A signal and not just the non-converted signal, of course. I think on the Mboxes you turn the "mix" knob to move between each signal.. not sure what hardware you have, though.
And mixsit, it's just an insert -- it won't be printing compression to the track as he records.. so no problem there.
The only thing you may have have an issue with is a small amount of latency.