why do you think itis dead, rock doesn't normally just die,
but to answer the question, you can add live rock, i would just be sure to add well cured(either buy locally or cure it yourself first if it is mail ordered, even if it says cured) and ad small amounts at a time, like no more then a 10 lb piece at a time to avoid throwing things too far out of balance and creating a mini or re-cycle
HTH