You can’t configure any speakers as down firing Atmos elements in the Sonos software. The Arc is the only speaker that has dedicated drivers for that part of the Atmos signal, and it bounces the signal off the ceiling. The Beam does not have dedicated drivers for that signal.
Any additional speakers, such as your in-ceiling speakers, if they’re driven by a Sonos Amp, can be set up as surround speakers, but not “Atmos” speakers.
Reports in this forum are that you can indeed hear a better quality out of the Beam when using an Atmos signal, but there’s not extra speakers involved, to my knowledge, and certainly no way in the software to define “Atmos downward firing” speakers.
Many thanks for taking the time to reply and expert advice