MIDI controller mapping

Does anyone know how to map an actual KEY on the keyboard to a controller? I’d like to map the “control” “command” “shift” & “option“ keys to buttons on my Machine so I can use the “hold down” functions without constantly taking my hand off the controller. It’s quite straight forward to map key-commands and transport commands from the drop down but I can’t see how to map an actual key. I’m in Nuendo 12.