You would be setting up that additional speaker in the kitchen as a 2nd room in the Sonos app.  It can then play it’s own audio or group with the HT setup you already have to play in sync.  The be clear, for music, the sync is perfect, as the system has the time to properly buffer the audio.  For TV audio, the kitchen speaker will play slightly behind the HT, as HT needs to play audio immediately to stay in sync with video.

So in short, yes, but if your kitchen and HT are in nearby spaces, you are going to get an echo effect with TV audio between the speakers.  If the two rooms are fairly well separated, not an issue.

Yes. I used to “group” a pair of Sonos Ones (or PLAY:1s? Don’t remember which, I sold them with that house) all the time with the Sonos PLAYBAR I was using back then.

There’s a slight delay between the two rooms, optimally at around ~75ms, but since the speakers were in a different room, and to hear both I had to stand in the doorway, it worked out just fine. And I could be cooking and still hear what was going on (usually football, but that’s just me). 

The Home Theater Room stayed 5.1, the “grouped” room was stereo in my case, but if you had only one speaker, it would be mono without affecting anything in the other room. 

You could increase BEAM’s lip sync delay to closer match the Group delay. While this would compromise lip sync with the TV, the two rooms will be closer to alignment.

Thanks for the quick answers - appreciate the help.

I use a One in my open plan kitchen. The Kitchen, dining area and living room form an C-shape, where my Beam, Sub and surrounds are on the lower horizontal plane of the C an the On is on the top horizontal side, probably 9 meters from the Beam. If do not notice the delay. 

Probably the time taken for the sound to travel from the Beam to the kitchen mostly eliminates the difference in lag!

Edit: lag on Beam approx 30ms

Lag to allow group sync approx 70ms

Time for sound to travel 9m approx 26ms