Yep, there's only a limited amount of analog axis controls at the moment. Pitch, Roll, Yaw, Throttle Absolute and Throttle Relative, if I recall right.
At the moment, all FS2_Open based games use fundamentally similar game engine, with minor differences in some cases. The only exception is Wing Commander Saga, which branched to their own version some years ago, but even that still has the same pilot code, and thus mostly the same controls.
You might be able to use your joystick's and throttle's profiler software to bind the buttons to certain key presses, and configure the controls that way. I assume Xpadder you mentioned does something similar.