I have never done this myself but it should be possible if you try changing the settings via the Mode Settings screen and the channel map option. You can read more about this in the KK2.1.5 manual You can then try to map your ch5/6 receiver inputs to pass through directly to outputs 5/6/7 on the KK2.1.5 board?
Let me know how you get on with this as I am keen to know if that will work.