Ever since I saw the reddit post about the neck ball dumbbell adapter to allow Marvel Legends heads on GIJ CS bodies, I've been a bit obsessed about it trying to think of non-toy hardware to use to act as an adapter. I've come across Tamiya remote control RC ball screws and connectors that have a ball sphere head (some with hex driver hole on top) and a bit of shaft below and then a nut and then screw threads. Not absolutely sure they have ball spheres with 8mm diameter. I suppose the nut could be filed a bit to be rounded to try to put into the neck hole which might have enough depth to accept the screw threads or the threads part could be sawed off. The Tamiya ball connector is similar but doesn't have threads so it's like a chess piece. I've never plucked out a GIJ CS neck peg dumbbell, so no idea what the diameter of the bottom ball is.
I've also seen spheres with a hole to accept a screw, but no idea if there are spheres with same thread size, but different sphere size.
Also saw something like the Tamiya ball sphere connector, but I think it's for belts where it's like a chess piece that accepts a screw to secure into a belt hole.
Also thought about if I could buy a 8mm diameter short aluminum rod piece and just try to file it into a sphere, shaft, and smaller sphere. I think that would be difficult to do freehand without a lathe.