It was possible to do it on PS3, but not anymore ever since the servers were shut down. When you enter a online lobby that is 10 player slots or more, traffic won't be on, Race Editor events can't be picked, and all car's windows will be fully tinted. Since both Murcielagos and the Cobra Concept don't have roofs, they're given one once you enter a 10 or more player slot lobby. Here's how it works: Create or join a lobby that is 10 or more player slots and switch to either the Murcielago or Cobra Concept. Then switch out to any another vehicle. Leave the Online lobby and go to the garage and switch to the Murci or Cobra. The roof should be on there for the cars. I did this on my Dub Murcielago years ago on the PS3, and my Murcielago on the Xbox also has a roof too.