Switch skeleton for mesh / Retarget mesh from one skeleton to another

I want to import the “Redshirt” character from the Infiltrator demo.
The problem is, that the mesh does not use the mannequin skeleton AND doesn’t use the same naming convention.
I have set up the humanoid rig on both skeletions.
I now want to change the redshirt skeleton to the mannequin skeleton, so that I don’t have to retarget all the animatons.

I can right click on the mesh and select Skeleton->Assign Skeleton, BUT the problem here is, that I’m missing bones (the bones that are named different)…