You can only add two surround speakers in a Sonos home theater setup. But you can group an Era 300 with the Beam setup. You will experience a slight audio delay from the 300 when playing TV audio from the Beam though. The better option would be to replace the One SLs as surrounds with a pair of Era 300s.

If you mean the same ‘Sonos Room’, you can’t add anymore surrounds - you can only replace the 2x One-SL’s with either 2x Era 100 or 2x 300 speakers. 

Era speakers can be added to other ‘Sonos Rooms’ in the same physical (actual) room in your home and ‘grouped’ with your existing Sonos HT setup too, which is absolutely fine for music audio, all will play in perfect sync, but you will likely encounter some latency (approx. 75ms) with those ‘grouped’ speakers when playing TV audio, as some buffering is required in a grouped room situation.

There are tools in the Sonos App to bring all rooms into sync, but you may then notice some lip-sync issues with the video on the TV screen.. Sonos room grouping is usually fine however when playing the TV audio to other physical rooms in the Home as the latency will likely not be noticeable when listening in a separate room and some distance away from the HT setup.