Sadly, it doesn't work like that.
The expansion packs contain new sample waveforms. Those organ voices you've mentioned are deep and rich (I have the Church pack for the S770) - quite different to the ones built in.
The S710 can only work with the waveforms that are already built into it. You can tweak the settings on those voices, but you cannot add new ones. If you took a voice file from a Tyros, it would either do nothing, or would play one of the built-in samples.