This tutorial explains how to use IK Rig and IK Retargeter to transfer animations to your own custom characters and prepare them correctly for Motion Matching. It covers setting up chains, poses, and root motion so retargeted animations work smoothly inside the Motion Matching system.
https://dev.epicgames.com/community/learning/tutorials/x40b/fortnite-epic-for-indies-retargeting-custom-characters-with-ik-rig-motion-matching-in-unreal-engine-5-70