Yes, you could power the in-ceiling speakers with a Sonos Amp and group the Amp “zone” with the Sonos Arc so they play the same audio. There might be a very slight audio delay when playing TV audio through the in-ceiling speakers, but it shouldn’t be too noticeable since the in-ceiling speakers are far away from the Arc.

You can’t add an additional pair of stereo speakers to a Sonos HT setup, not for TV audio.  You could setup a second room and group them to your HT setup room, but that comes with limitations.  For streaming audio playback, the audio will play in sync.  For TV audio, the Arc will play audio immediately to sync with the video, but the second room (your speakers in the back) will be slightly buffered, creating an echo effect.  

There are two other options you might want to consider.  First is that you could setup the amp as surround speakers to play surround for TV audio and full music stereo when streaming audio.  Perhaps that gets you where you want to be.

The second is to just get a single Amp, no Arc and connect 2 pairs of speakers to it.  The first pair to bookshelf speaker or in wall in the front by the TV, the second pair for your ceiling speakers.  That will get you stereo for TV and music in the whole room in sync.  You can’t change the volume between the two pairs of speakers without a separate volume control switch though.  Since you stated that are not interested in atmos or surround sound, this might be the best option for you.