Include extra motor on existing frame

Hi guys, quick question. If I want to include an extra motor in a already existing frame (like a bimotor configuration for the arduplane) which parameter would I have to change to enable that? Do I have to change anything on the code or this can be set on mission planner/qgroundcontrol. How about add an extra servo? Would that be easier to be done on the arudpilot or maybe px4?

If you are looking for unsupported frame types, I would start here: