Transmission fluid

1997 Oldsmobile Cutlass supreme 130,000 miles.
My son is losing transmission fluid and it had a slight leak in the pan which we sealed and it still is losing fluid. The other day I checked his radiator expansion tank and there is an oily substance in it. Could the transmission fluid leak into the radiator? Is the radiator cooler internal in this car? And can it be fixed or do you have to replace the radiator? How difficult is it to replace ourselves? Engine is a 3.1L with A/C