It is possible but you'll have to do some re-wiring. The existing wire can be used, but a new wire must be run so you can have a switch for both light and fan. This could get a little tricky if you try to do it yourself. I'd probably call a good electrician to do this unless you think you can do it. One thing, rather than put another box in for two switches, I usually put a piggy back switch in, two horizontal switches in the same box, saves some work.