I would recommend you do a little research on I/P transducers - very common over the years as a way of converting 4 to 20 ma current loop to 3 to 15 psig or similar pneumatic output.
I - current
P - pressure
I/P, I to P,
You might discover how it's been done commercially and work your way back.
If you have an air supply you can construct a means to vary the pressure - you have a tiny control valve - a needle valve that allows an amount of air to flow - near the discharge is a small orifice that bleeds some of the air - the balance goes to the load. The pressure is varied based on needle valve position. You might operate the needle valve with a coil that responds to current flow.