Beam is already front left and right, plus centre channel. You can add two rear surrounds and/or Sub. 

You can use any matching speakers for a front right/left stereo pair.

While you can’t add them to the Beam as additional front speakers you can Group the Stereo-Pair Room and the Beam Room and have all play together. May give you a muddy stereo image though.

Technically, I’ll disagree. That extra pair of speakers are not “front” speakers, nor do they get sent the front right and front left signal of a 5.1 signal. Instead, because they’re a separate “room” in the Sonos software, they get only a standard stereo signal consisting of all data from the TV. Definitely not “front” speakers in any home theater setup. Nor, in that case, does the sound bar become a “center” only speaker. And as Stanley points out, there’s a slight delay between the “home theater” room and the second “stereo” room, which would muddle the entire sound image.