Yamaha purposefully gave the PSR-S950 a limited 64MB flash expansion so that people who wanted lots of bells and whistles will go for the Tyros 4.
On Yamaha's sell sheet they say that the initial offerings of expansion packs are styles and voices from around the world - ethnic music.
But if Yamaha started offering a great grand piano, killer organ sounds, SA horns in their expansion packs many people would want more than one and to not have the limitation of having to load each expansion pack.
So my thought would be to
1) Get a tablet and a 2GB flash card
2) Partition the flash card with 64mb partitions
3) Open up the keyboard
4) Remove the 64mb flash memory
5) Replace it withe the 2GB flash card
6) Find a way to control which partition the keyboard reads from the tablet
Then you can install as many expansion packs as you wanted, and if you wanted to change expansion packs you can just choose which partition you want from the tablet.
It's probably impossible. It would void the warranty. It's probably not worth it. Just a thought.