“If the machine’s software supports adding balls to a game already in progress, a tournament official will add a ball to the game in progress and the affected player will complete their game.”
Only modern Sterns (SAM and Spike) can add a ball during the game. Enter the Menu, go to the “Balls Per Game” setting and set it to 4 balls during a 3 ball game.
Here is the list of Sterns that DO NOT allow a ball to be added (i.e. Whitestar):
High Roller Casino
Other manufacturers that do not support adding compensation balls during a game:
Heighway Pinball (except Alien can add compensation balls)
Jersey Jack Pinball
Bally/Williams (do not use “buy in” button to add a ball at the end of the game, many WPC games with buy-in support have rules changes beneficial to the player on the buy-in ball)