It seems like a waste of an H9, and MIDI potential, but I believe (from memory) that you can pull it off. Set the H9 Output to XMT (transmit), and the H9 + 3 keyboards to the same MIDI channel. CTL.XMT needs to be On. Set the expression pedal [PED] to transmit CC7 or CC11 under XMT CC.
I was also going to mention the Source Audio Reflex, on the higher end. It would really open up the independent control over those 3 keyboards, plus additional unique features, & 128 storable presets.
A more cost-effective solution would be to use your existing expression pedal with something like MIDI Solutions Pedal Controller. Small inline box; powered by MIDI. If you’re lucky enough to find one, there was an ancient Anatek device that’s similar, but more limited in scope.