You cannot add a One and have it be in the same “Room” name as the Beam and surrounds as seen in the Sonos app.  You could group it with the Beam and surrounds, but It will be delayed by 75 ms when playing TV sources, which will cause an echo if it is close to the Beam and surrounds.  So it’s not going to improve the sound if you use it in the same physical room with your current setup, but you can certainly add it to the system to use in another room.