They are really hard. This is one of those situations where "if you have to ask how to do it, you probably don't have the right tools to do the job".

If I really wanted to drill a hole in one I would thrown a few into my wood stove, get them red hot and let them cool down very slowly buried in ash to get them softer and easier to drill.

You could grind a flat spot onto one side so the drill won't "slip" off when you try to drill it with a carbide bit.

The best choice would probably to just attaching something onto the ball via welding or brazing instead of trying to drill a hole. I assume you are trying to attach the ball to something.

Best of luck

Eddie, I am surprised to hear that pinballs are case hardened and not through hardened. Great photos.

If it is the situation that most pinballs just have a thin case on them it would be easy to just grind through the case and drill with a standard drill. You can avoid the expensive and easy to damage carbide drill.

I have welded onto ball bearings, it works. But you can get some very interesting stress fractures as things cool down. It normally works out OK if you only are welding to small part. I have made up a few stakes for hammering metal onto for shaping by welding on a piece of 3/4" square stock that I can hold in a vice. They work great, super smooth and hard surface to hammer onto.