This is how I introduce new chickens to my old flock. I have been using this method for many years with great success. The photos will tell the story. I have made this small enclosure inside my chicken run where I place food and water for the new baby chickens. On the one side, close to the fence there is a gap just big enough for the new chickens to leave and enter the enclosure. The older chickens are too big to follow them into the enclosure. They learn very quick to use this escape and it works well until they are fully integrated. This method also ensures that they always have excess to fresh water and food.