I did this to a 2 ball when I was in highschool ( WAYYYY Back ) tried a few different times, seems not all pool balls are of "quality" materials.
The best composition I found was from a commercial table (bar, bowling alley, pool hall, coin op tables ) balls for home use have a wierd sand like core to them.
The commercial ball had a solid core ( solid as far as I drilled anyway)
filed a little flat on the side of the ball I wanted the hole in and drilled, tapped it to 3/8-16
the flat worked well with a locking nut to help hold it on the shifter. A Heli-coil or timesertwould help greatly.
DISCLAIMER: I do not advocate the piffering of balls from above mentioned establishments, At the time I happend to know the owner of an amusments co. that had cases of damaged balls and equipment. and was able to get them from him.
Kevin