No, you can’t. The older birds will bully and can even kill the chicks. Never mix birds of different ages. If you are adding birds to an existing flock they all need to be adult birds ideally 20 weeks +. 18 weeks as a minimum.
Also, you should never add one bird to an existing flock as you run a high risk of bullying, add new birds in a pair as a minimum. They will have a barney to decide who is at the top of the pecking order, but that usually sorts itself out quite quickly.
When you add new birds try to give them plenty of space to run away from each other so they can have a chance to run away if they lose out.