You can target an angel, or demon, or guy-with-cape, from the Unreal animations.
You don’t need custom bones, because while you should map each bone in the unreal skeleton to your skeleton, you do NOT need to map every bone in your skeleton to the Unreal skeleton.
They will work just fine – they’ll walk and jump just like the standard skeleton.
However, the wings/tails/capes won’t animate, but will stay in default position.
Custom bones does not solve this problem, because the standard animations won’t know how to animate your wing/tail/cape.
Instead, you solve the problem by adding new animations that only target the new bones, and provide a blend space that lays those new animations on top of the standard animations.