Can I map a keyboard controller to the guitar/synth mix dial, or does it have to be a floorboard?

Any Midi controller that can send control change (cc) should work.

Ty. Can you describe how I can do that?

I can only describe how I did it originally with an Akai LPD8 controller. It is a small unit with 8 pads and 8 knobs. If you open the interface tab on MG2 you can see a slot to assign a midi controller, click here and a list of any controller you have connected to your USB port should be seen. Click on the one you wish to use (if more than one) then click on midi learn in the interface panel. MG2 will highlight any control wich can be midi controlled. Turn the knob or tap the pad you want to use on the controller then turn off midi learn and you will see that parameter change as you turn the knob on the controller. I hope this helps.