All things considered, I don't think that it would be worth trying to convert that into a 7.1 system. I don't think that it would be possible to create a true 7.1 system with it. You could run two more surround speakers and have them output the same sound as the existing rear surrounds (so, a "fake" 7.1), but that wouldn't necessarily be inexpensive or even worthwhile.
Just my opinion.