If you have +4.5 build, you don’t have to use Epic Skeleton. You just would like to keep their hierarchical bone structure, but you can have different ref pose, and use retargeting for targeting to new mesh.
“CREATING A CHARACTER FOR THE UNREAL ENGINE MARKETPLACE (AFTER 4.5)
In order to ensure maximum compatibility, both in the content samples and in the marketplace, we provide an asset - Rig’/Engine/EngineMeshes/Humanoid.Humanoid’ - and your skeleton should set up using the rig.”
Make sure to configure “Rig” set up. In the latest, it changes to “Select Humanoid Rig” and you just have to make sure you map all the nodes to the target.