KingTumTum101 hit the nail on the head.
- You need to import your model and skeleton in to UE4 first so that the skeletal mesh can be created.
- You then import your animations on to that skeletal mesh created in the step 1 above.
- Once this is done, you can then re-target your animations to a different skeleton (or the other way around).
The retargetting basically creates a duplicate animation targeted to the new skeletal mesh and does not override your original. I think this is what you post is asking about … 8-}