Just hanging your 6 volt Lamps off the exisiting relevant GI lighting string.
I would not do this is your game is all or mainly incandesant bulbs. If you have LEDS then go for it.
12 VDC unregulated use J138 Pin 2 is 12VDC and pin 3 is ground
Check with multimeter before using and do not use the 12 VDC regulated supply unless you want your Machine Toasted.
My WH20 has 17 additional GI leds and the overall load is still way less than using the factory incandesant lighting.