You can group two Play:1s to the Playbar setup, but you will likely experience a slight audio delay from the Play:1s when playing TV audio. Streaming music will be in sync though.

NOTE: The Playbar already includes the center, front left, and front right channels. And you can only add two surround speakers to a Sonos home theater setup.

No, unfortunately not. The PLAYBAR already contains the right, center, and left speakers inside the ‘box’, there is no way to separate them so that the Sonos soundbar becomes only a ‘center’. 

The Play:1’s you mention cannot be ‘bonded’ to the Playbar HT room, but they can be ‘grouped’ with the Playbar, as outlined in this link:

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

Note however any rooms ‘grouped’ with a Sonos HT product will have a ~75ms minimum audio buffer delay when playing TV audio and so will likely play that audio out of lip-sync with any video on screen and cause an echo-like effect when the speakers are in the same physical room. Note this only applies to TV audio, any Sonos Music audio will play in perfect sync across all the ‘grouped’ Sonos rooms.