Usually an expression pedal is continuous, and a sustain pedal is a switch. (Of course, people with real pianos know that the sustain pedal isn't really an on/off swtich at all, but I digress.) It's unlikely that your expression pedal would work in the sustain input.
Heck, I have a Kurzweil PC-88 and a Yamaha expression pedal. The Yamaha expression pedal doesn't even work in the PC-88's expression pedal jack. It seems that Yamaha and Kurzweil use different specs for the resistance and linearity of their pedals. Well, the pedal kind of works, but no amount of messing with curves and soldering on series and parallel resistors will fix it. I need to find a micro-potentiometer that fits the Yamaha pedal, but has the Kurzweil values to make this work.
I should just buy the Kurzweil pedal and sell the Yamaha on ebay.