Grouping the additional 100s for TV use will result in a delay/echo that many find objectionable. For music streaming it would work well.

I'd consider a pair of 300s so you could do Atmos music using just them.

Or just have a stereo pair set up as an additional “room” (since a room is just a label to identify a speaker or set of speakers) and use them for music play. 

to clarify, I dont really use the Sonos for music, mostly TV and movies

You cannot add front speakers to a soundbar.  All soundbars already contain the front Left/Center/Right in a single enclosure. 

I can pair my era 100s stereo

Sure, but that is not adding front speakers to a soundbar.  You can pair them and group them, but for TV you will get an echo, and all channels; fronts, surrounds, and Atmos will play out of the stereo paired 100’s ruining any Atmos or surround effects.

Earlier you said you don’t listen to music. Why do you want an additional set of stereo speakers? It’ll interfere with a 5.1 sound system if you run them together (as well as being out of sync for a video source). You seem to be contradicting yourself.