I’m not quite sure I follow…
If you want to add something as a real Fortnite prop (ie it wobbles when hit, gives resources when broken) you need to create a Blueprint and do some additional setup (e.g. what resource to spawn), as described in the docs.
If you just want to add the mesh into the level as a non-interactive mesh, you can just drag and drop the mesh from the Content Browser and it creates a FortSMActor.
I certainly agree there is a mix of important and esoteric option when importing meshes though!