This add-on pack includes the custom balls:

STRIKE KING
MELEE
MASTERMIND
DIRTY GOLF
LIGHTNING
HURRICANE IRENE
THING
COUSIN ITT
UNCLE FESTER
LIGHT BULB

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 FarSight Ball Pack ...