although this may not be exactly like sustain it is pretty close to the same effect, hope this helps and if your unclear of how to do this let me know

*note i have obviously tried this with the sustain knob, but it has little/no effect*
actually you could also try linking your sustain pedal directly to the instruments "release" knob, unsure if this is possible but worth a try....use right click on the knob > connect to controller, then press on your pedal, it should be automatically picked up by the instrument, or at least look at the drop down list for midi controllers and see if it is in the list.