You can set up either on the H9 or in H9 Control what parameters are mapped to the expression pedal and over what range. So, I think you just need to map the Mix parameter to go from 10 – 30 on the expression pedal.
Here's a tutorial video showing how to do that in H9 Control:
It's also explained in the H9 Control manual under Settings / Help.
If you check out the H9 user manual, that explains how you can do this directly on the H9.
