Sorry, I’m no expert with ArduPlane but have been following several forums on the quadplane development. I actually use PX4 and have successfully built and flown a few conversions. Having said that, I’m hoping to point you to the right direction. @GregCovey has done a few and you could look at his Ranger setup.
This is done by assigning a switch with Q Flight Modes to FBWA.
If you want to give PX4 a try, you can look at the preconfigured VTOL airframes and probably the QuadRanger would be compatible with your project. Samples of my builds:
QuadRanger FPV view or Ground View
RVJet
Good luck.