No.  The Arc already contains the Left-Center-Right, you cannot add more.

Thanks. But is it possible with any other Sonos speaker as center speaker? 

No. All Sonos soundbars are 3.0, not just center. One (SL), Five or Symfonisks are always rear speakers in a Sonos surround set up.

No, Sonos sells soundbars, and like every other soundbar made, they contain the L-C-R in a single enclosure.  You can use an Amp with third party speakers up front.  That gives you a Front Left and Right, and a phantom center.  But there is no way to use an actual center with that configuration, 3rd party or otherwise. 

If we’re lucky they will let us add L & R wide, then the ARC can maintain 3.0, but you could still add additional channels. I suppose that’d make the setup a 9.1?

Yeah I wish we could add a few more speakers to the setup, I would like to able to add two rear ceiling speakers to act as the overhead channels, Sonos needs some rear speakers that support overhead channels like the LG SN11 or Samsung Q90r.

I wish Sonos hinted at things in development, or like Hulu’s community forum, allowed idea voting, which they (if it got enough votes) would acknowledge it was “pitched” to superiors.