This tutorial shows how you can import Mixamo animations into Unreal Editor for Fortnite and convert or transfer the Mixamo animations to Fortnite characters. The IK Rig Retargeter makes the process more seamless and simple than ever. Instead of retargeting bone by bone, it is now done in a chain. This same procedure also applies in Unreal Engine.
https://dev.epicgames.com/community/learning/tutorials/JL5a/unreal-engine-mixamo-to-fortnite-character-animations-using-ik-rig-retargeting