Re targeting mesh to another skeleton with extra bones


I’m trying to reassign a mesh’s skeleton for a particular mesh to another skeleton which has a weapon and camera bone, but for some reason when I right-click mesh > Skeleton > Assign Skeleton, the new bones just do not work. I hover my mouse over the bone whilst looking at the mesh’s new skeleton hierarchy, it says: “This bone exists only on the skeleton, but not on the current mesh”.

All I want to do is make a skeletal mesh use another skeleton, which has the same bone hierarchy as it’s default, but with 2 extra bones added onto it, a weapon and camera bone.

Any help on this would be fantastic, I’ve been searching around google etc and I can’t seem to find an answer