Songtrix

Songtrix is basically a Midi editor, so we can import Midi only.

But can Songtrix take the MP3 or WAV of a song and convert to MIDI first (as a preprocessor) before importing into the 'guts' of Songtrix for editing and other functions?

Songtrix hasn't this option for the moment and I guess that it will never have it because Audio to Midi conversion is so hard to realise. You can look around of course to find a standalone converter, but it seems to me that it will be hard find a good one.

What Andre said :)

This is a very specific function, and one very difficult to do properly.  

Likely to be never part of Songtrix, where our focus is on creating original music, and producing quality output.

I suggest that you focus on finding a piece of software that does this single task very well, then you can import the midi files it produces into Songtrix.