+1 what @soix said......
You can add HT/surround to your existing system, without changing anything in your existing system. I've done this several times over the years when I needed to have a combo/HT system that did not negatively impact my 2-channel sound. Now, admittedly, you will compromise the potential overall quality of your HT sound quality by using the HT receiver to power the center and surrounds. However, it will not affect your 2-channel sound quality at all. Matter-o-fact, you can have the HT receiver turned off when you are listening to strictly 2-channel audio.
You can add HT/surround to your existing system, without changing anything in your existing system. I've done this several times over the years when I needed to have a combo/HT system that did not negatively impact my 2-channel sound. Now, admittedly, you will compromise the potential overall quality of your HT sound quality by using the HT receiver to power the center and surrounds. However, it will not affect your 2-channel sound quality at all. Matter-o-fact, you can have the HT receiver turned off when you are listening to strictly 2-channel audio.