The topic is pretty old, but just sharing my sloppy way of doing it. Basically if I need to add more and more animations from the fbx file, I just click Create Animation asset within UE5, then I name it exactly like the animation I am going to add. Then I press the newly created empty animation and click Reimport - specifying the FBX with that one animation I want to add. It only sees one animation, so if there are multiple - I add them one by one - re-exporting an FBX with only one animation. That’s not the most efficient way but helps if you don’t want to reassemble the asset you’ve already invested much time into