Yes, you can! I do agree with the others that using a floppy on a Motif would be a little primative, but if you just wanted to store midi files it will be fine.
Buy the Yamaha Midi Data Filer. They sell it at Guitar Center or musiciansfriend.com and it's around 300-400 dollars. It hooks up via midi and it can store and edit midi to floppy discs.