You can use the Pre-outs for the rear L/R channels and send it to a cheap-o DPL processor. It will then mix your L/R into a L/C/R. You then send that to a secondary amp to get your 3 rear channels. Of course... if you have a old DPL receiver you can skip the part about sending it to a amp. This has been done for ages on DD/AC-3 material to get 6.1 until the 6.1 recivers came out.