er, yes............its called an expression pedal, erm........
All you have inside an expression pedal is a pot with a pedal attached to it. This means that any synth with an expression pedal input can have a knob added to the front panel wired striaght to the expression pedal input.
I have a knob added to my Casio FZ1 sampler that i think can control filter cutoff via the expression pedal input. I haven't turned it on for about a year and a half so i'm not entirely sure.