Is there some way that I can use the keys on my MIDI controller keyboard as secondary controls to the normal keyboard controls, so that, say, I can use a certain key for W, another for S, and so on? I'd find it pretty cool if you could have velocity of the keys set up to give the speed you move at for the movement controls, although I'm not entirely sure if this entire idea is currently implemented, so I wouldn't be surprised if it wasn't either. 

I don't think so, but you can try to use it as it was a controller: you go into controls settings, press the keys and see what happens.
If it doesn't work you can search if it's possible to do it via a plugin or some external software.