>>292631537
Name the new mesh group vertex as one of the bones you want it to follow.

If the new mesh has no group vertex yet, make a new one and assign the vertices as that new thing.
Apply an armature modifier.
Then merge later for optimization.

This would merge the group vertex of both meshes. If you have a hat for example on "Head" bone. It will merge on the old mesh's group vertex as long as the group vertex's name is "Head"