Can this be machined? custom center-bore adaptors

Why do you need it? Automobile wheels are stud piloted. They center on the studs. The clamping force from the elasticity of the stud holding the wheel against the face of the hub is what keeps it there. You can use a wheel with center hole bigger than the hub - I’ve done it. You just need to be careful and tighten one lug all the way down by hand and not just let it hang there. You could just let it hang there and hit it with the impact and the taper on the lugs will pull it up into place - it just might mar the wheel a little bit.

If you really want one that thing would be tough to machine, you’d have a lot of waste. It would be better to make it in two pieces and weld or braze them together rather than hog all that out.