Configure a midi keyboard with the Audya4 is very simply.
You find in page78 of the Audya Main Manual.

Press: MENU -> MIDI

You can save the MIDI settings in five Preset,
assigned to the USER buttons from 1 to 5:
General: General Preset
Master1: Master Keyboard Preset
Master2: Second Master Keyboard Preset
Accord1: MIDI accordion Preset
Accord2: Second MIDI accordion Prese

A keyboard with USB midi I dont know. But I think is possible.