This add-on pack includes the custom balls:

FLIGHT 2000
MUSTANG
GHOSTBUSTERS
STAR TREK
WHOA NELLIE
RIPLEY'S
HARLEY DAVIDSON

Once unlocked and selected, Custom Balls can be used on any table. To change your pinball to be a Custom Ball, select Extras from within the Options of any Table Menu.

More details on the official game page of Stern Ball Pack 1 ...