There won't be any ".1-.9" settings for Sustain. In the Midi spec, sustain is an on/off feature...not a Zendrum limitation, just the way Midi works.
What you really need is to fix the note length in the synth module. Often there are settings for attack, sustain, decay, and release. By messing with each of these, you can, with some practice, really tailor a sound to your liking.