Only one centerback speaker

First off, I would like to say congrats on the receiver purchase - it's a nice one. To get true 7.1 surround sound, you will have to run another wire to another speaker. Any of these listening modes that you want to get are most likely using the capabilities of these 7 speakers. If you were to hook up one speaker to the two rear surround channels, you would be defeating the purposes of having that extra "speaker" and not really be gaining anything. If you really do want to access the 7.1 listening modes, you could always hook up another speaker and just hide it behind your TV, depending on your setup (essentially tuck it in a place where it wouldn't be heard, close to the receiver so that you don't need a long wire run).

In answer to your specific question, there would be ways of doing this, but just not all that advisable - you'd be posing a hazard to your speaker and/or the receiver. Like I mentioned, I'd recommend going through the trouble of running another wire and hooking up an additional speaker for the rear surround. If you're going to be using a Bluray or HD-DVD player, it'll be worth it. Hopefully this helps to somewhat answer your question.