I have never done this but was told it is possible.

If you save(if possible) a stereo recording of your song, send it out from some outs, into your processors and record that singal again. Then you have 2 versions of the same mix. One processed and one unprocessed.

I guess you'll have to figure out how to do it.

I hope this helped.

Looking at the D1600 (and the D1600 mkII) on the Korg website, there is NO provision to patch outboard gear like compressors or EQs to the mix bus (or ANYWHERE on it, for that matter), so that won't be possible. You CAN patch compressors and EQ' between the Mix (L/R) outs and an external CD recorder (for instance). But the bigger question is this:
WHY? There are decent "insert" and "master" processing functions within that box that will do basically what you are wanting the outboard stuff to do. You are also dealing with data-compression schemes that aren't really favorable for a "big studio" sound...You need to learn to "live within the box" on this one, I believe, James.