Jim, No one here can positively tell you that you can add an additional fixture and not overload the circuit w/out being there and performing a load calculation and determining what other fixtures are also wired to that circuit. Given the fan/light combo does not draw a great deal of power, as long as NO HEAT LAMP is incorporated, most will agree that you PROBABLY will be able to add to that circuit w/out adverse consequences. Keep in mind that if the bath light circuit is the same as for the bath plugs, and you have a curling iron, hair dryer, and clothes iron using that circuit as well, you may want to reconsider adding another fixture. You may want to determine how many watts are currently being drawn by that circuit, convert the total to amps, and then decide if the additional load requirement will push the current load rating beyond it's capacity. You could be maxxed now, but we here have no way of knowing for sure.