You can’t if they’re ‘bonded’ to the Arc - you would need to unbond them first. You might find it easier to add another Sonos One (or perhaps a Sonos Roam portable speaker) to the room instead.
It is possible just to play music to the Arc only, by switching off the surrounds without unbonding them, but I guess that’s not what you were looking for.
As another thought, you can shift the music audio more towards the rear speakers too using the Music Level slider control in ‘Surround Audio’ (in the Arc ‘room’ settings). but again that will play audio on the pair/sub, rather than just the one speaker.
My own choice would be to perhaps add a Roam to the room setup instead.