I've been researching this topic lately and i don't think putting surrounds on top of your existing surrounds will do anything buy give you more volume and muddy your soundfield. If you have room maybe you could move your couch up a couple feet and mount your new side surrounds to the wall or ceiling at about 190 degree angle from your listening position (slightly behind).
I guarantee someone on here is gonna tell you to paint or hide those speaker wires and i guess that anal retentive person would be me.
