Assigning MERGE / REPLACE to a hard button (function key)

I was wondering if it would be possible to assign the functions MERGE and REPLACE to a physical key on an ONYX controller or NXK (single click on a rotary encoder button). I use these functions very often and as I mainly use the keypad, it would speed up things a lot by not needing to reach for the virtual button (popup) on the (touch)screen with a finger or a mouse on PC.