any torque converters i have seen flushed had a drain plug in them.
now with that said, i don't have a clue about the converter flush machines of today that are out there in transmission repair shops.
as for drain plugs, in the past, i have added a 1/8"npt plug just by drilling and tapping the converter.
used heavy grease on the drill and tap, then stuck the shop vac hose over the hole and turned it on for a while.
i followed this up by standing the converter up so the drain was on the bottom, then filling an empty squeeze bottle with kerosene, and squeezing it into the converter snout, repeating until the kerosene ran clear.
followed this by filling converter with atf, "swishing" converter around and then draining atf, doing this a couple of times until the atf was the original consistency drained as it was when poured into the snout.
this method worked good, and i had no converter failures doing so.
however, this was way back in the mid 70's to mid 80's that i did this, so with re-man converters being [relatively] cheap these days, i would pop for a re-man.
just my personal life experience.
your mileage will vary.