Introducing new chicks is always a difficult task, but not impossible. Ensure the chicks are old enough (and big enough) to integrate with the new flock - I would not introduce 2-week old chicks to an established flock of mature chickens. Wait until the chicks have all of their adult feathers and are of an appropriate size to fend off any potential attacks by the established chickens. Slowly introduce the new members by placing them side-by-side with the existing flock (use chicken wire or another see-through barrier). You want the existing flock to be able to see, but not touch, the new chickens. After this has been done and no aggression observed, remove the barrier and keep a sharp eye out for aggression. If aggression is observed, you will need to separate the new chickens and try again. Keep trying and eventually, the flock should integrate. Please note that there will always be some aggression, as the introduction of new chickens will cause the pecking order to be re-established. Having a rooster can sometimes be a help with this, as the rooster is always the "alpha." In an all-hen flock, the dominance of the top hen is sometimes challenged.