I would suggest using an automation track and automate the SF2-players ENV/LFO [volume tab] "release" knob, for example if you want to sustain a few notes but not others then automate the "release" knob to raise in value for these chosen notes. you could make the envelope of the knobs automation fade in/out too instead of just min/max.
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.