Hi Richard --
This kind of question always seems to come up.
Deep down inside, voices depend upon the fundamental sounds -- waveforms -- that are stored in the keyboard. The actual voice programming is built on these fundamental sounds. Users can edit many of the important characteristics of a voice program (e.g., attack, release, brightness, etc.) and save the edits in a user voice.
If the SX900 has the same waveform as the T5, then Yamaha may have already provided the T5 voice as a preset. As long as that findamental sound is available in the SX900, then it should be possible to move the T5 user voice on which it is based.
If the SX900 does not have the same waveform, then anything else is an approximation to the T5 voice.
The SX900 does not support SA2 voices, so they are completely off the menu.
There are lots of geeky details in voice programming, but I hope this simplified answer covers your concerns.
All the best -- pj