According to the U.S. Department of Agriculture, it is okay to refreeze chicken that has already been cooked. Freezing stops the growth of bacteria and mold.
Leftover chicken can be refrozen within three to four days, as long as it has been stored in the refrigerator during that time. Cooked chicken should not be refrozen if it has been allowed to sit outside of the refrigerator for longer than two hours, or longer than one hour if the temperature is above 90 degrees Fahrenheit.
It is possible to reheat cooked chicken from its refrozen state, but the USDA estimates that it takes about one and a half times as long to cook frozen chicken as it does to cook thawed chicken.