Can Era 100s be used in the front to enhance a 5.1 setup?

Not in a standard Sonos setup. The Beam provides the three front channels. 

Some folk have done it, and say they like the result, but (a) they will be playing all sounds as a stereo mix, muddying the centre channel and surround effects, and (b) there’s a delay which will create an echo effect. 

In any Sonos HT system you can only attach/bond two rear channel left/right surround speakers and either one, or two, Subs (one must be a G3/G4 Sub depending on the setup), or alternatively you can use one Sub Mini.

The front channel left/right speakers are built-into the main Sonos HT products, such as the Beam (gen2), Arc etc. so you cannot add speakers to replace those front channels.

You can ‘group’ (not bond) Sonos rooms together however and this works great for music playback, but note that any rooms grouped with a Sonos HT for playback of TV audio will have a 75ms audio buffer delay - if the grouped speakers are in the same room you may notice an echo effect across such grouping.

There are tools in the Sonos App however to help address this sync issue - see this link for further information:

https://support.sonos.com/en-us/article/tv-audio-and-video-are-out-of-sync

Note: the ‘TV Dialog Sync’ feature will of course buffer the TV audio output and that ‘may’ lead to you noticing lip-sync issues with the video display on screen.

Hope that information assists.

It would be up to you to find out if this really would really “enrich the front soundstage” though. And as I understand it, Trueplay will not work with such an “enriched” system.