No, not possible. The Beam is the 3 front channels (like any soundbar).

Lots have asked for such a configuration for many years. 

Thank you for reply, nik9669a.

Actually, when I called sonos support I was told it was possible.However, I was asked to set it up in an environment with a real machine, but since support is only available from 9am to 5pm, Monday to Friday, I couldn't do it without taking time off from work.

However, no matter how much I check the sonos app myself, it seems impossible.

I mean, I was one of the lots.

You can create a second room and have the Eras set up as a stereo pair. Then you can group them, so they play the same source. But, the Eras will be playing 2.0 stereo whilst the other room is playing 5.1 so it will destroy the 5.1 effect. In addition there will be a 70ms delay for tv signal to the Eras (or any other speakers Sonos offer) resulting in an echo that will further muddle the sound. 
So you can have a 6-speaker setup but only as two rooms, and certainly not a single surround system. 

Thank you for the detailed explanation.However, all the other rooms have each stereo systems.If I want to use One or Era100 as rear speakers, is Era100 better?

They’re newer in design and manufacture, which likely means newer electronics (a different chip set). They’re marketed as better sounding, I think, but you’d be better off going to a local retailer who carries them both, and determining for yourself. 

On the other hand, I’m of the opinion that there really is very little ‘action’ in a surround speaker, so I continue to use the Sonos PLAY:1, which I feel plays fine as a surround speaker. Where this might change is if you plan on using your surround speakers for music purposes, setting them to ‘full’, in which case I could see an argument for ‘better’ (and this is relative to your ears) speakers. 

Depends mostly on the content, with older content outside gaming using surround speakers just for effects, but that is changing.

Gaming has generally made good use of surround speakers for a long time, including ambient music playing.

From a TV/Movie perspective it has been noticeable that more recent streaming has also started using the surround speakers as part of the soundtrack playback, not just for frequency range limited effects. How that matters is an individual decision, but surround speakers are increasingly being sent more than just effects these days.

I mainly use the speakers for watching movies and listening to music.
I only use these Sonos speakers with my FireTV device for "movies, TV shows, YouTube, etc."
I think it has enough performance to be used in a small room.
I live in Japan.
As you may know, the rooms in Asian homes are smaller than those in Europe or America.
The main audio system is located in the living room.