You don’t have to rig to bones, as long as they are individual meshes that have basic transform animations (translate, rotate, scale) then you need to link all of the objects to a single root object (can be a dummy) and then export using FBX. In the FBX export options enable the Bake Animation section. When importing to UE4 make sure it’s set to import as a Skeletal Mesh and then check the Import Rigid Animations box.