Hello, You can export animations as fbx and you can import it to Blender without any issues. But you may use some plugins for animation controllers for Mannequin.
I am not sure about the best practice between Unreal and unity but I may suggest an alternative. First import the Unity model to unreal , then retarget the animations to the new model and export it . then directly import the animations to unity. Theorically i might work :)
