You already have the maximum speakers allowed in a home theater ‘room’, in the Sonos software.

The Arc has front right, center, and front left, along with upward firing channels for Atmos, it can not be used as a ‘center only’ speaker. There is no way to add additional ‘front’ speakers. 

You could potentially expand your system by replacing the Sonos Ones with those Sonos Era 300s, they would help with the Atmos signals that you may be getting.

I take that back….a bit. You could add another Sonos Sub to your home theater room, as long as neither Sub is a Sub Mini, and one of them is a third generation Sub. 

Thank you… The original plan was to replace the Sonos One’s with Sonos Era 300’s, but before I move the Sonos One’s to a different room, I was wondering if I could enhance the front stage. 

Unfortunately, Sonos doesn’t currently offer that option. It’s been a request since they originally released the PLAYBAR, but so far, nothing. I’d love it too. 

For music, the Ones would help up front, just Group them with the Arc. Gets you a bit more volume and if well positioned might improve the stereo image.

I’d likely move the Ones to another room or rooms.