Unless you happen to have a large drill of the correct size and a 1/2 or 3/4 pipe tap laying around, it'd be a lot cheaper to go to a plumbing and piping supply house and buy a bushing of the dimensions you need. The current plug is probably either 1 1/2, 2, or 2 1/2" pipe on the OD. Bushings are widely available with any of those OD threads and most any ID pipe thread you could want.
Even if it has to be double bushed to get to the size you want, $10 or so should cover the bill. A decent quality 1/2" NPT tap and the corresponding drill is going to cost you at least 3X that much in import stuff, and $50 or more for domestic tooling.
One problem with attempting to drill and tap an existing fitting is that a lot of fittings on the market today are really poor quality from a metallurgical standpoint. Some of the worst are fittings used in high volume production products where cost rules over all else. You might be able to drill and tap the fitting you have, or you might rip the end off a $20 drill in finding out the fitting has hard spots in it.