Can I add in-ceiling speakers to my Sonos arc?

No. Any Sonos home theater room requires up to a soundbar as the front speaker (L/C/R), or an Amp (as R/L, with a ‘faux’ center channel). To that you can add one Sub mini, or up to two Subs, and a pair of matched surround speakers (Era 100s or 300s, Fives, Ones, or the IKEA speakers). 
 

There is no method to integrate into the Denon universe…in fact, Denon is paying some royalties to Sonos, I believe, as they lost a case for stealing copyrighted tech to Sonos. I don’t think they’ve changed that tech yet. 

So it is not possible to install ceiling and side speakers using Sonos soundbars?
Thank you very much for your detailed explanation.
Thank you very much.
I will consider combining Polk and denon to achieve this.

Please define ‘ceiling’ speakers. 

There are many folks who use ceiling speakers as surround speakers, but I wonder if you intend height field, rather than surrounds. Like the extra directional speakers in the Era 300s, and the upward firing speakers in the Arc. Both of those directional speakers are used to generate a height ‘channel’ when the Sonos system is being fed an Atmos signal. 

But the maximum number of discreet (separate) speakers in a Sonos system is 5, as explained above. However, the number of channels/discrete speakers in those boxes are much more.

Sonos is designed for ease of use, and good quality sound. If you’re more concerned about number of discrete boxes, each with its own specific use, then another system would be a better choice. 
 

I am currently using a JBL bar 1000.
I asked this question because I wanted to configure a surround sound system that exceeds this without using an AV receiver.
I really wanted to use NAKAMITI DORAGON, but I found out that it is difficult to use in Japan, so I asked my question here.
Thank you very much for your detailed and careful answer.
For now, I think I will use the DENON AVR-X6700H and think about the configuration.

Hi @akira1244 

Welcome to the Sonos Community!

An Arc with a pair of Era 300s as surrounds and a Sub will deliver 7.1.4, although some of the speakers are virtualised (side) and depend on reflections (side and height). You can have Dual Subs if at least one of them is Gen 3 (the current generation), but as the Subs both play the same track, it’s not entirely accurate to describe it as 7.2.4. Note that Dolby Atmos uses an x,y,z co-ordinate rendering system rather than discrete channels, however.

Sonos soundbars only support adding 2 surround speakers at this time (but please don’t take that to mean it will change - it may not). It’s worth noting, however, that Arc has 11 drivers while each Era 300 has 6 (5 of which are active for optimum surround sound). Add Dual Subs (each with 2 opposing drivers) and there are now a total of 25 drivers working in concert to supply you with immersive sound.

Hard sell? I don’t know what you mean! 😇

I hope this helps.

In my opinion, people worry too much about the “7.2.4” older style designation.  Once you get to Atmos, it doesn’t apply.  When mastering Atmos, unlike regular surround, the sound engineer doesn’t say “Send bullet sound from RF to RS channels, center dialog on Front R-C-L”, etc.  Instead, they place a sound in a room coordinate, not in a particular channel.  Then when you play the soundtrack in your room, the Atmos decoder decides where in the room the sound is played, according to the coordinate, and which specific channel(s), according to the room layout.  

Along the same lines, people just assume Atmos is always better.  If you do not have a specific location where you’re going to be listening to the audio, then a stereo or basic surround system may be a better option may be the better option.  Atmos is a terrible option for a sports bar type atmosphere, for example.  Then you have a high emphasis on the quality and volume of the audio equipment while completely neglecting the acoustics and layout of the room.  Sonos speakers do try and accommodate and take advantage of the layout in your typical home, but there does come a point where if you’re trying to improve the audio in your room, you need to change your room, not your speakers.