That's going to be the least expensive way. Heck, even keeping it, and buying the 3802 will be cheaper than trying to add the necessary stuff (none of which is inexpensive).
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.