Assuming you mean drum kits, I recommend you start with Mike McCue's SD tutorial here:
http://www.harmoniccycle....09-SessionDrummer2.htm The tutorial is based on SD2, and he uses a custom skin that makes it look like some alien VSTi, but it's still applicable to SD3.
If you're talking about programming your own non-percussion instruments, you'll want to look into creating SFZ files for the SFZ player, or maybe using a 3rd-party softsynth sampler. There are some freeware ones available.
I don't know what you mean by "usb roll up piano". You mean like a membrane-type keyboard that sends MIDI? I don't know if such a beast exists.