You can freeze pretty much anything you want. However if the casserole is made from chicken which has previously been frozen then the official advice is that it should not be frozen again.
In general so long as food is frozen soon after being cooked, and is properly defrosted and re-heated thouroughly. (In Scotland the food in a commercial food premises must be reheated to at least 82°C
In my personal opinion it is not possible to adequately reheat frozen food in a microwave!!