In my opinion I have found that it is easier to introduce your new additions to the flock by putting them in a closed in area close enough to your flock so they can get used to being around the new additions. What you need to use to separate them is a homemade/fenced in cage that is either made of chicken wire or some type of wire they can't escape from. I made a homeade fenced in temporary chicken home that has a wood bar to roost on at night and it has a cover for the top so they dont fly out.
It works really well for a temporary little home so your flock can get used to them. This may take a week or even weeks. Depending on how your flock reacts. After awhile, if your flock hasn't shown aggression you can go ahead and put food out for your flock to get them distracted. Then you can open up the door and take out your new chickens. BE AWARE that the chickens will most likely have to do a pecking order. Let them go through it. After they have all seen where they are on there "leaderboard" then they will all go back to their normal day business.