Even if there's not a mapping Mixxx has a "MIDI Learning Wizard" (I forget what they call it) where, assuming it speaks MIDI, you can plug it up, choose an action, and then move the control for that action and it will figure out how to wire it up. For simple configurations that don't require scripting (ie. no setting LEDs on the controller or what not) you can get a fully functional setup for a controller Mixxx has no knowledge of without writing any JavaScript or XML at all!