Your flock may get territorial of their food and water dishes since they will feel more dominant in the run. Add at least 1 additional food and water station inside the pen when you plan on bringing in your new chickens. Keep the new stations away from the existing ones so your chickens don’t fight one another when you introduce them. After your chickens are familiarized with one another, try letting the new chickens into the run while the rest of the flock is inside the coop. Let the new birds explore the chicken run for 10-15 minutes so they can get accustomed to the new space. Keep watching the chickens to see how they interact with the space to make sure they’re comfortable in the new pen. It doesn’t matter if you raise your chickens in a pen or free-range. Always let the new chickens explore the area first so they get used to the space. After your new chickens have spent a little bit of time in the run, let the rest of the flock out of the chicken coop so your birds can intermingle with one another. Make sure to watch your birds the whole time to make sure they interact well with each other. Let the birds act naturally and don’t try to force them together since they could get aggressive. It’s normal for your chickens to get into small fights with one another since they need to establish a new pecking order to determine which bird is dominant. While it’s normal for your chickens to get into small fights with one another, they can get more aggressive if the fights escalate. If you notice any birds that are bleeding, take them out of the run so they don’t get any permanent injuries. Take the chickens that attack any other birds and keep them alone in separate pens so they don’t hurt one another again. Keep the birds in isolation for 1-2 days before reintroducing them. Distract the chickens with treats so you can break up any fights before they even happen. After your chickens are comfortable around one another, they may join the rest of the flock in the coop at night. If the new chickens follow the flock into the coop, let them stay there for the night so they can continue familiarizing themselves with one another. If the chickens return back to their own pen or crate, then allow them to get comfortable on their own.  It may take 4-5 times for your new chickens to feel comfortable with the rest of the flock. If the chickens return to their own crate, then you can transfer them into the coop at night so they wake up with the rest of the flock.

Summary:
Set up separate food and water stations for your new birds. Let your new chickens into the pen first. Release your existing flock to greet the new chickens. Isolate any birds that are bullying or attacking others. Let the new chickens join the flock in the coop.