Jick,

No they can not. With either of these keyboards, you are stuck with the sounds which are stored in the instrument's ROM (Roland Sound Canvas SC880 in G1000). You may modify the effects which are applied to a built-in sound (and store them as part of the perf. memory), and you can layer two of the built in sounds (and use velocity switching). There are over 1000 sounds in the G1000, so that you can find a multitude of sounds of any type of instrument (there is probably about 20 trumpets, and as many strings). However, you can not add new sounds like you can in Yamaha or Solton (or make new programs like in a Korg i30).

Regards,
Alex