Yes, it’s possible, but the volume setting will affect the ON-state only. You will have a volume jump by switching off the H9, doesn’t matter you are in Relay- or DSP-Bypass
Take a look at H9-Control’s menu Pedal Setting – Assign MIDI CC messages to pedal functions – Output Volume (or Input Volume)
Just set it to some CCxx. Now you can control this CC by ES-8
