VERY unlikely that you could add a 3.5 drive after the fact. I'm sure that would involve a major overhaul, IF it could be done at all. That's surely a segment of that motherboard or a piggyback chip you'd be lucky to find, period.
I'd say hook up a Mac/PC and go with a lbrarian such as Emagic's Soundiver or buy a Yamaha MDF2 or 3. A Roland MD series sequencer would also handle a sys-ex handshake that would allow you to do this.
PCMCIA cards can still be had from Korg, but to save some bucks, I'd also suggest that you peruse
www.roguemusic.com and
www.ugbm.com for 2nd-hand ones. With SmartMedia taking over, many folks are ditching their old cards, so prices such as $20 per are not unheard of. Good luck widdit!