I'm thinking I was once told the way to do this was to first drill the part to the same size as the hex & chamfer the entry. Then take a hex wrench, use a small burr to grind a recess in the end forming a hex "cutting edge". Then drive this make-shift broach into the drilled hole to cut a hex profile. Make sure there is bottom clearance for the chip.
Never tried it, but it sounded good at the time..