5.1 to Pro Logic (I and II) Conversion Possible?

[GK6475]

So, I was thinking that it may be possible to "invert" the Pro Logic I & II decoders somehow so that they instead bring a 5.1 signal down into a Pro Logic output. I have a receiver that only accepts Pro Logic I for surround sound, so would it be possible to create a configuration text file that downmixes 5.1 into Pro Logic I, maybe also PLII for those with PLII receivers? My sound card will output a surround sound signal even if I only have one stereo 3.5mm cable plugged in, so that's not an issue.