I don't think you could "attach" it. You would have to rebook with the candlelight package and cancel the old reservation.
And I do not believe the credit allotment changes for the package booking. You are still paying for the meal. You are not paying extra for the CP.
They may ask for a credit card to hold the reservation, the way they do for the Fantasmic dining package. The card would be charged if you don't show up, but not otherwise.