if you get the KX drivers you might be able to get it to fake it by making a mixer or 2 in 6 out in its dsp, so that way you get your stereo info out of ALL speakers
eg you would set "line in" to goto splitter that has 6 outputs, then you shoudl be able to route that to where you need
there is no magic way to convert stereo to 5.1, other ways involve just adding a slight time delay and reverb for the back speakers, but that always sounds like crap.
using the KX drivers would be the best option in you just wanted the same thing out the back but still have the ability to have 5.1 play as 5.1.
check the sound card compatibility list there before you install, some usb sound cards are supported, some are not