If you already have Era 300’s as surrounds bonded to your Arc you cannot add the in-celing speakers to work as a cohesive unit controlled by the Arc.
You can add a Sonos Amp to power the in-ceiling speakers as a separate room. When playing music you can group them to the Arc (with surrounds) to fill your space with more sound; although it might be overkill.
The in-ceiling speakers can be used for ambient music sound when using the Arc home theater (and or grouping) is not desired.
Also, know that TruePlay tuning (if your device is iOS) will not work for the Arc and in-ceiling speakers at the same time. You will have to TruePlay the Arc and surrounds as a room and then the In-ceiling speakers as a room. If you add a sub to the Arc it cannot be used with the In-ceiling speakers.
Together Everyone Achieves More = T.E.A.M.