Welcome to BYC!
The first step to mixing new birds with an existing flock is isolation. The new birds should be kept separate for approximately 30 days to be sure they won't be introducing an illness or pest to the existing flock.
The next step is "look but don't touch". That means the 2 groups need to be able to see each other but not be able to peck and hurt each other. You can separate your coop and run space with chicken wire or poultry netting. Let them have a chance to get used to each other 3-4 weeks. Then it's time to take the divider down and let them mix, be sure to have extra feeders and waterers out so no one gets chased away from food/water. You can also let them get to know each other free ranging where the new birds can run and hide from the existing flock. Some people use crates or other types of pens during the look/don't touch time but since you're not getting chicks, the larger new girls will need room.
Hope that helps.