No way to set up faders AFTER the fact, but definately before. Here's why:
The FW card transfers up to 18 tracks of audio at a time (16 from onyx to CPU, two from CPU to onyx) and unfortunately there's no way to assign to specific channels through outputs, only inputs. Which, because most people mix on their PC mixer anyway, is usually not a big deal. So everything I do in works on the way in (I can ride the gain and fader knobs no prob) but output control is not yet an option.
You don't need a FW card for your CPU, just a FW input.
Oh, and you are correct, the faders are not automated on the Onyx.
Let me know if I can help out any more. I really love this mixer.