Hmm. Page 66 of the owners' manual seems to suggest that you can adjust Attack, Decay, and Release for the "real time" keyboard parts. Nothing about sustain, but depending on the base sample, the sustain may be determined by how long you hold the key (or how quickly you strike it.)
In general where the parameters attack, decay and release determine the time...
Thats where with sustain the parameter eddits the level as the time of sustain by default is set bu how long you hold the key...