No. 

Whilst you could set up additional speakers as a new Sonos room and have them physically in the same room as your Beam, it won’t help. The Eras would have a slight delay, resulting in an echo effect. The delay is inherent in the way Sonos achieves multi room sync’d sound and is unavoidable. 

Edit- the post above was posted while I typed.

There is a something you could try.
You can go to the room Settings and ‘Remove Surrounds’. Then set the Era300 as a stereo pair. 
Then when you are playing TV on the Beam, go to the Grouping screen, tick the Era300 pair and Beam, and press Apply to group them. The TV sound will play through those. 
The danger via this route is that there might be a sound delay of 75ms or so that could sound like a slight echo. (The Beam usually connects to the surrounds through its own wifi to keep the surrounds in sync - grouping them as I have suggested might not be 100% in sync).

But you could always try it. You’ve nothing to lose - you can always unpair the Eras and put them back as Surrounds.

A cheap AV amp would be better suited for your parents, e.g Yamaha RXV4A £400 (uk) plus a basic speaker bundle (£300 Monitor Audio Mass) is less than a Beam and a pair of Era100’s.

The Yamaha amp has a full stereo mode which turns the input into stereo and plays across all speakers, it even does streaming and a real FM/DAB radio tuner built in and way more flexible.