Yeah...it probably could be done...i'm just thinking about what you'd need on both ends. You'd need a SWIM controller on one end for the floppy drive (or something similar to a SWIM controller, anyway), and circuitry on the other end that adapts said SWIM controller to run on the USB bus and makes the computer recognise it as a standard USB Mass Storage Device. It'd be a mad mad wild hack, but if you've got the money and the smarts, it'd be doable.
When I said, "Can't be done", I should've elaborated some more...what I meant is that there's no prefabricated solutions that allows the above to happen, you'd have to either build it yourself, or pay someone else to do it.