My PSR900's won't arrive until Thursday but I'm sure my answer will be correct. Most of the Yamaha keyboards can receive aftertouch from a midi controller which features aftertouch as an option. The 900 cannot have aftertouch when played from it's own keyboard, but if a midi keyboard connected to the midi in jack has the aftertouch ability, the PSRS900 will receive this information and it's sound will respond to the kind of aftertouch transmitted from the midi keyboard.
You cannot expand the voice set and when you connect a sustain pedal and play a super articulation sax for example, the breath is heard when you press down on the sustain pedal. So, if you use it between phrases, it will sound like a sax player taking a breath before he begins playing the new phrase.
------------------
George Kaye
Kaye's Music Scene
Reseda, California
818-881-5566
www.kayesmusicscene.com