2150cn - adding duplexing

Just bought a 2150cn which has no duplexing built-in. I meant to get the 2150cdn, which is exactly the same printer but with duplexing. I really don't want to have to go through the hassle of boxing this beast up again and shipping it back. I imagine there just some small part that allows for the duplex feature on the 2150cn...

Does anyone know where this part can be purchased? I have combed through the "parts and accessories" section and cannot find anything....

Thanks.