Technically it is possible, but it's an advanced topic, and it is outside the scope of the addon, you'll need to retarget back in Blender to modify it in the NLA. In Unreal you can Retarget and Bake the animation to our "matching" RIG2U5 Rig, export the .fbx. Import the .fbx in Blender, delete the fbx from the outliner. Select our RIG2U5, set all controls to FK (legs and hands), use a retareting tool (like Rokoko addon retargeter), Select the Unreal FBX action in the action editor, retarget the Unreal FBX baked animation to the RIG2U5 rig (assuming it's all good from the fbx export from Unreal), and finally bake the result and push it to the NLA, to edit. (mind you'll have a keyframes on each frame animation and not the soft curves, you'll then need to clean curves, etc). This process assumes you know how to retarget and bake in Unreal, export the .fbx, retarget back in Blender, and bake that action into the NLA. The scope of RIG2U5 contemplates editing animation from Blender and send it to Unreal. It's forward->oriented not backwards. All though it is possible, it requires advanced retargeting on both softwares