If you just put him in with them, there will be an adjustment period. Depending on ages of the birds, it may take a bit longer. If he's not a mature rooster and these are mature hens, you just stuck a teenage boy into a flock of middle aged women. If he's also mature, he will eventually earn their respect. Chickens don't do well with change or other chickens just suddenly appearing. They are very territorial.
Personally, if you are having that much trouble with hawks, I wouldn't rely just on a rooster to protect my hens. He may be a good warning system (a dominant hen can to the same), he may lead the way to cover (again, something a dominant hen could do), and he may even give his life protecting them. But then what? You're out a rooster and would have to go through the integration thing all over again if you want another one for protection. You may want to consider putting them in a covered run for a while until the hawks give up and leave.
What is their range area like? Is there plenty of cover for them to go hide? Shrubs, bushes, pallets on cement blocks... anything they can get under if a hawk comes around would be helpful. One day when I was out with the flock, a buzzard flew over. (No threat to the chickens, but they didn't know that.) I looked around and *POOF* there was not a chicken to be seen. They disappeared so fast, I didn't even see it happen. In my opinion, that's better protection for them than a rooster.