Adding post fader send to a mixer?

I've just got my first Arduino project working (more-less, still a bit buggy on the code side). It's a USB Midi Electronic Drum Kit.

Now I'm toying with the idea of modding my SA.3 mixer. Right now I'm believeing of adding some MIDI functionality, just buttons for now though I might try making the faders/knobs send MIDI CC, which hopefully won't be much trouble. The other mod I'd like to do is add a post fader send. Has anyone tried something like this? Right now the big problem is I don't know where to start looking for a place to tap into, and a few pointers on what kind of circuit I'd need to build would help a lot too.