No, a ‘room’ in a Sonos setup is a set of speakers, you don’t have a choice to use a subset of those speakers without re-defining the ‘room’.

Essentially, to use the surround speakers separately from the sound bar, you need to un-bond them and set them up as a separate room. Then, when you want to use them as surround speakers, you need to re-bond them. All possible, just more effort than most people are willing to do. 

You can adjust the “bias” so that most of the music is played through the rear speakers. Go to Settings, System, roomname, Surround Audio and move the slider control for Music Level. 

Just to briefly add to what nik9669a suggested - it is best to set the surrounds to ‘Full’, rather than ‘Ambient’ for Music Playback too.

It is possible to ‘toggle off’ the surrounds in the App, but not the other way around where the Beam is left ‘muted’.

One reason I personally wouldn’t use such a feature, is I prefer the stereo music soundstage to be more out in front, rather than behind my listening position, but appreciate that some say they find the Beam front section of the soundstage narrower due to the shape of their room/rear surround speaker placement …and it can slightly diffuse/muddy the stereo separation for them. 

One other way (perhaps) is to add another pair of Sonos speakers to the room, set out in front for music playback …and I understand some users here on the forum, including myself, have opted to go that route instead.

Thank you for your support. I understand there is some small tweaks to do from your replies above. 
The reason I ask is that I want to use it in 2 different ways. First one is movies with surround. But the other one is that I at the same time want to use the back speakers as normal stereo, without the beam, when listening to music.