It is generally a bad idea to 'just add a couple of extra speakers'. If you have a used decent 5.1 processor on a home theatre system, you will have seen the timing and phase hoops you need to jump through to set the system up correctly. This is to correct for the time taken for the sound to travel from speaker to your ear. In a simple 2 speaker setup, the speakers are approximately equidistant from the listener and the effect is not significant. Have one pair of speakers closer than the other, and without time/phase correction it sounds a mess.
if you want to experiment, there was a technique popular years ago called 'rear channel Hafler' it tried to synthesise a rear channel from the stereo difference signal. Google 'hafler speaker connection' and have a look. Personally I was never impressed, but it might work for you.