You can create something similar to the UDK prefab by turning you group of objects into a blueprint asset. There is a button in the details panel when you have multiple actors selected. It’s still not the same as the prefabs in UDK though as only things that can be made into a component will be added to the blueprint (Geometry, volumes, etc will not work). (Edit: Just realized that maybe this was the case with UDK prefabs as well)
Not sure if it is possible to export all of a blueprints components as a single mesh though so it may be a bit more work to get the LOD made. You probably also need to make some blueprint code to have the meshes switch to the single mesh LOD (This can be reused on all your “prefabs” through inheritance though).