That in itself is a snag. Any metal that will graft to bone is non-ferrous and Magneto should've had no effect on his skeleton.
I don't think its grafted as much as became infused within the bone, making the bone fully metallic.
Either that or the metal strips became porous & leeched into his bones.
Apparently, the adamantium is in his bone marrow, so whenever he regenerates, the adamantium does too.