Is it okay to use superposition in conjuction with source transformation?
If you mean in the sense of:
Use a source transform or two to change voltage source + series resistor to current source + parallel resistor and then apply superposition to find the voltage / current across some resistor that WASN'T TRANSFORMED, then yes.
Remember, you can do source transform because you can treat a resistor and voltage/current source like a black box (i.e. change the black box with something else that has the same output / resistance and have no impact on the rest of the circuit).
Separate names with a comma.