You can only add two surround speakers to a Sonos home theater setup. And the Arc already includes the center, front left, and front right channels so adding two more front channel speakers to the Arc is not supported by Sonos.

You can put more speakers in the room of your house but you can’t add then to the Sonos Home Theater Room.

If you put more speakers in the room of your house you will have to give them different Sonos Room names.

You can Group the Sonos Home Theater Room with the other Sonos Rooms you have created.

BUT the sound will have many issues and will likely sound horrible.

• The Grouped Sonos Rooms will be delayed from the Sonos Home Theater Room by 75 MS.
• The Grouped Sonos Rooms will play a stereo or mono mix of the source and disrupt both any surround or Atmos sound the Sonos Home Theater Room is trying to create.

Arc, 300s and one or two Subs is the best you can do. 

Thank you very much for your replies. Had the feeling I wasn't able to. 

Is there a way to increase the sound range?  I have a similar setup with the difference being in ceiling speakers instead of the 300s.  I use Apple TV as the source.  My problem is my room size is a bit on the large size with an open kitchen right behind the TV room.  Any sound coming from the kitchen massively disrupts the sound in the TV room.  Is there any solution to this problem? 

@jerry262 

To answer your question all is said above… makes no difference to the maximum of speakers in the setup what kind of rearspeakers you use. 
You just could replace the in ceilings with Era300 to get better Atmos sound, but the whole stage won’t increase that much with other formats or music. 

I tried adding a pair of era 100s and it definitely helped but I couldn’t control the sound with my Apple TV. I had to use the Sonos app to control all audio including muting, which is a nonstarter for my household. So forgive me for repeating what may have already been answered. Is there anything that can connect to my existing setup to increase the amount of sound without just blasting the volume? 
the in ceiling speakers are nice for ambient surround effects but they don’t really add to the overall sound in the room. 

If the Era 100s were not being controlled by the TV (the signal is fed by the Apple TV to the TV), then they were not ‘bonded’ as surround speakers to the Sonos soundbar/Amp. 
 

To be clear, a home theater can comprise:

Front: A Sonos Soundbar (Right, Center, Left channels) or a Sonos Amp (Right, faux center, Left channels)

Surrounds: any matched pair of Sonos non soundbar speakers except the mobile speakers (no Roams or Moves)

Subs: one Sonos Mini Sub, or up to two standard size Subs (one must be gen 3)

You can group as many rooms with that Home Theater room that you want, but the other rooms will always be delayed by 75ms from the Home Theater room. 

@jerry262

Just to say… for music mode sources there’s no problem and grouped rooms will increase the sound stage and your overall listening experience easily. 
But for tv mode sources via cabled digital input there are the mentioned limitations. 
There groups will…

…in case of multichannel sound cause a loose of strict channel separation,

…need to compensate the 75ms delay via „tv dialogue synchronization“ option,

…need a very strong wifi connection to avoid dropouts,

…need the Sonos app, or voice assistant, or special hardware to remote control the whole group.

In the end… imo not very easy to manage. 

Sonos told me the 100s could not function in that way so I sent them back. Seems a bit odd to me that there isn’t a “standard” solution for this. 
when i tried adding the 100s to my “TV Room”. It merely created a second room with the same name. 
apart for the fact that I couldn’t control the audio with the Apple Remote like I can with the rest of the setup, it was pretty nice. The additional sound, while perhaps not true surround did what I wanted, there’s just no way I could deal with having to use sources to control the audio. 

@jerry262

If that was just ok and satisfying for you, you could solve the remote issue as said via voice command (eg. Alexa groups) or an Ikea smarthome hardware remote.