Add new bone to skeletal mesh with multiple animations

I have multiple hand skeletal meshes in my project, (human hands, gloves etc) and I’m trying to add forearm and upper arm bones to one set.
The new bones would be a parent of the hand bone.
The problem I’m having is that I have 50+ hand poses all set up with the old skeleton. If I import the new skeleton the animations no longer work.
I could retarget the animation to the new skeleton but this means that I will have duplicate animations, one set for the new skeleton and one set for the old and would like to use the same set of animations on all skeletal meshes.
Animation data for the two new bones is not required and would not be included in the imported animation sequences as the bone transforms would be set in the animation blueprint.

Is there any way I can use the same animation sequenceson both skeletons? or is my best bet to go through and add the two new bones (unskinned) to the old animation sequences?