No, neither the Playbase, Playbar or Beam will support Dolby atmos.  Atmos requires physical up facing speakers and likely other dedicated circuitry.  

Agree…

but in theory if you mount 1-2 Sonos ONE on the ceiling and manage them through the Sonos app (exactly like you can do today with surround speakers) you get an Atmos effect that is way better than any soundbar

Unfortunately I think this option might not be commercially attractive for Sonos, that is currently focused on selling the new ARC soundbar 

The Playbase has an optical connection, and that cannot carry the necessary DD+ stream that is a minimum for Atmos. There isn't the remotest chance that your existing Playbase will ever support Atmos.

That’s actually not true, depending on the dimensions of your room.  You want audio to come from above, but not to be able to pinpoint that the audio is coming from a specific speaker.  So if your ceiling is low and/or the ceiling speaker needs to be fairly close to the listening area, you are better off bouncing the audio off the ceiling.  I  think it’s fair to say that for most rooms in a home, the lack of stereo separation in a soundbar is a bigger issue than upfiring speakers.   And honestly, if you have the facility to mount Sonos Ones on the ceiling, why wouldn’t you wire ceiling speakers?