you can not really machine the drum inside for more plates. the plates lug into the side grooves and the groooves only go so deep.
it is best to machine the piston down for max capacity. there are also thinner plates and steel plates to increase the clutch count.
Since the fwd drum is not a shifting clutch , thinner plates will not over heat
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.