I just picked up a Source Audio Reflex expression pedal and it works just fine with the H9. It has 3 analog expression outputs (2 at 10 kohm and 1 at 50 kohm) as well as USB and Midi out which can also be used for expression control.
I have mine set up with the 50 kohm analog out with TRS cable and it works fine.
One benefit with the midi out is that you could send expression over midi to the H9 and use a separate 3-button aux switch.
https://www.sourceaudiostore.net/product/reflex-universal-expression-controller
