If they are being quarantined away from the existing hen, they must NOT be allowed out to scratch in any place that the current hen will access during that time. The whole point is to prevent them passing any bacteria/virus/parasites/etc. to her, and taking turns on the same ground is one way to spread things.I wouldn’t keep them in there for more than a couple days without letting them out in a pen to scratch .
If the current hen is only allowed out in the backyard, it might be possible to let the new ones scratch somewhere else (maybe in the front yard?)