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.
I got a decent one from ebay fro £8 but that was a fluke i think. Bear in mind that in order to be as irritating as possible there are two plug polarities used by Yamaha and Roland etc
I think the more popular method is the roland one but i'm not sure you uses either way round.