No problem! First of all, I only used unity package (I checked "FBX" and "ALT FBX" but they need additional modifications for importing to unreal engine, which I'm not good at). Secondly, I used a paid plugin called "UMotion" in unity, which can convert *.anim files with a skeletal mesh container to a fbx file. This fbx file can be imported into unreal engine with a model. Thirdly, UE5.4 can easily auto retarget this fbx file to any standard skeleton mesh in unreal engine.