A "tap" in this context is a special drill-like thing used to put threads on the inside of a hole. In other words, drill a hole in the shaft (don't go too deep) and then tap the inside of the hole so you can bolt on a new knob.

The other option would be to have a local repair shop see if they can replace the potentiometer with a new part. That's probably the best way to go.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8