If there is availability then yes, normally you can add it to your existing booking. You can try to add it online yourself or, you may need to call. Also, in order for it to be applied, everything needs to line up exactly so it's not always as easy as it seems. In that past sometimes I have easily been able to apply a discount and other times it has meant a lot of time on the phone.
My existing reservation was giving the CM grief due to rebooking it a few times so if there had been something that worked for me I may have needed to book a brand new reservation anyway. I'm currently waiting for a callback from Disney IT to see if it can be resolved. Current wait time for Disney IT is 160 mins! Thankful for the callback option.