Not only allowed, but required. You must have ventilation, unless there's a window you can open.
I say that, imagining a completely separate shower room. If you're simply describing a large glass shower door, then the existing bath fan might neet the HVAC code requirements.
Another possibility is a remote-operated fan. That is, a fan that is located somewhere the exhaust ducting, and not actually mounted on the ceiling of the shower.
If you do have a separate room, remember that the switches need to be outside the shower area itself - in the changing area, or even outside the room.
