SONIC FOUNDRY HAS IT
You did not specify what kind of equipment you have to do your mixing and I am not familiar with the equipment you have, maybe because the prices of 5.1 encoders/writers/burners are still extremely steep. But, as long as you have at least an 6 bus mixing board or have enough outs, you could possible mix 5.1. The problem is to encoding them.
Sonic Foundry used to have a software encoder to mix Dobly Digital 5.1 that used to sell for one grand U.S. They have stopped marketing them, maybe because the market is not yet ready. But you could possibly call or email them. You may also post want ads it BBS's around the internet, and maybe, find one in eBay.
Good Luck.