Re: How can I add 2 more rear speakers to my 5.1 system
You can find a speaker selector with impedance matching and independant volume control (so you can balance the speakers) to split the surround channels. But that won't get you 2 more discrete channels (7.1 with surround and surround back channels), just 2 split channels (5.1 with 2 speakers per surround channel). I'm ignoring the front height channels here.
Alternatively, if your BluRay player has 7.1-ch analog outputs you can use your AVR to give you the 5.1+ht channels, and a separate 2-ch amp/receiver to power the surround back channels. This would giver you discrete surround back channels but without the benefit of any additional processing for those channels. And this would not help if your source is only 5.1 (there is little 7.1-ch material out there now).
I don't think either of the above are great solutions, but I don't know if any other way to get your cake and eat it, too.