This is already supported. In H9 Control in the presets screen, if you look under the More menu, there is an option to assign the output level to the expression pedal. You can also globally assign the expression level to the output level for all presets under Pedal / General Settings. You can do this on the H9 as well. Instructions for doing that are in the latest version of the H9 manual.
Does that override existing expression pedal assignments though? I would like to add output level changes to other parameter changes, for example: in Crushstation have the expression pedal control gain from 15-30 and output volume from 0dB to +6dB at the same time.