Is this something that you do by combining two switches into one channel inside the transmitter configuration, and then using that one channel in LibrePilot?
That is the way to do multi-switch flight mode switches.
Edit: Ah, you might mean that these other firmwares need you to be in the a special flight mode for the arming switch to even work? I can imagine the FMS on one channel, and the FMS+Arming on another channel, with the lowest FMS position and the lowest arming switch position both required for it to arm.