A rough way could be while in FBWA or FBWB mode to connect the elevator servo to the radio receiver instead of connecting it to the AutoPilot elevator output.
You 'll need a radio receiver with both PPM and servo output , guess that some Frsky receiver can do the trick.
Of course since the AP “think” that he control the pitch servo , several features will not work, for example anti stall, and results might be unpredictable.
@lucamax makes a good point. You could probably just reassign the servo2_function to RC passthrough so that you don’t have to worry about any special reciever.