Yes it's legal

It is perfectly fine.
And legal.

 Almost every used ball I’ve ever owned has had this done, and only ever had the fingers crack out on one of them.

 I’ve got an original black hammer from 1987 that I plugged and redrilled the thumb on in 1988. Moved the thumb 3/8 inch and redrilled, leaving a partial plug.

 Still throw it at spares, and it has never given me any problems.

Besides cracking out, the junction between the ball itself and the old plug can develop a ridge that can cause skin damage.  I recommend that holes drilled into an old plug be filled with either a slug, a sleeve, or a finger grip.  --  JohnP