I think it's like this. Start off with a hole, fit the ball then press the metal in till it reforms and locks the ball in place.
The hose barbs (brass outlets), and the hole it fits into, have slight taper, then when the brass goes in they tap it together, and the tapers hold against each other.
You may be able to tap the body, but may only have room for 1/16 npt.
Might be easier to look for other petcock type, and or weld in a new bung.