I want to turn potp optimus prime into a super ginrai, i will be using siege frenzy as the powermaster, and then use removable 2 double barreled gun as the smoke stacks for super mode, the problem is i need to add 5 mm holes on the grill of the vehicle mode meaning the back of the base robot, one on each shoulder, and one on the front of the super mode, if anybody knows how add 5 mm holes please tell me.

 

Drill them? It's just holes.

Get a 5mm drill bit.

Where it might be tricky is does the part have enough supporting thickness or such? If not, you'll have to add something... Glue some plastic, put a thick layer of epoxy or such... BUT you have to figure out how much to not create a situation where it blocks other parts, especially with the transformation.

 

Just to add.. one solution that works for me on some projects was simply to glue some earth magnets on both parts (making sure about the polarity first).

Easy to do, no damage to anything.

 

Like they said, a 5mm drill bit (look up a drill conversion chart if you have drills in other measurement units).
If for some reason you can't use drill bits (with some effort you can work them by hand if your doctor doesn't allow you to use power tools), another thing I've used is Lego Technic/Bionicle with the hole:


The hole is just 4.8mm, but if you shave the inside until you reach the outside of the circular border around the hole, it will be 5mm.
Replacing a panel or part of it with is not always possible, but it is a good option to consider.