This one takes me back to the days when folks were hotrodding their early analogs!
Off the top of my head, I'd say the chances were slim. Modern digital synths are basically hard-wired from the git-go and changing to a different disk drive would be not unlike trying to change the ROM soundwave set, which only Kurzweils will do as far as I presently know. Its usually a job featuring diminishing returns.
I'd say track down a tech at
www.roguemusic.com or ugbm.com and ask. You could also try
www.korg.com, but they're less likely to be up on or interested IN modifications like this. Never hurts to inquire.
I'd recommend that you consider either a Yamaha MDF1 MIDI Data Filer for some expansion capabilities or go the whole computer route. With the latter, you could use a Zip drive, which is cheap, tough, standardized and offers a good seek time for accessing yer bits.
Otherwise, consider going with a computer and a librarian program such as Sound Diver or Galaxy. You can get a one-instrument version of SD for $99.
I'd love to hotrod MY fd that way; its SO darned close to perfect...! Please post whatever you discover, if it gets that far.