What do you think that frozen cooked chicken in the supermarket is? It's cooked chicken which has been frozen. It's ok for you to do the same thing, assuming the chicken is not bad before you freeze it.
It is not OK to add raw chicken to a cooked chicken casserole. It would be safer to cook the raw chicken separately, then add it to the cooked casserole when reheating if you will be eating it all at once. Otherwise, keep the freshly cooked chicken separately to avoid reheating it several times.
First, do not re-freeze chicken whether cooked or not. Second, if you are cooking a fresh chicken make sure it is fully cooked before you freeze any left-overs. Warming a cooked chicken is one thing, cooking a frozen partially cooked chicken isn't going to work well.