I;ve seen em try and use a hole saw and a drill... but more often than not it just ruins yer hole saw. I have heard that if you use common mustard on the hole saw it prevents the teeth from burning up,, worked for me on a damn dull drill bit once.I'm purdy sure they do make a knockout for use with a hydraulic punch but not for the old threaded,,, use a wrench type.
What gauge you tryin to punch a hole in? Greenlee knock outs are good for 10-14 gauge steel and they make a 4 inch..
It the steel is too thick a good drill and a hole saw will work just be careful cause the drill can knock the crap out of you or twist your wrists pretty bad. HANG ON TIGHT!! lol If you go this route be sure to put plenty of lube on the saw and let it do its work dont force it cause it will cut fine on its own.
to help keep a holesaw cool while drill wet a newspaper and fill the inside of the holesaw with the paper ring of the water out the paper like a towel, not a good idea if anything is hot around the area