bathroom wiring

You can't do it if there isn't a neutral wire in the box. Right now you have either one or two hot wires and two switchlegs (the wires which go to the light and the fan) For the receptacle portion of the switch/receptacle to work there has to be a wire that goes from the receptacle to the breaker box.
One may already be there but I doubt it. You can bring one down from the light fixture and wire both light and fan to the switch. That means, of course, that the fan will run everytime you turn on the light.

Joe



Hot wire ......... fan wire
....... SW````light wire

l

-----Rept ```neutral wire