Reservations are essentially caveats to a state's acceptance of a treaty. Reservations are unilateral statements purporting to exclude or to modify the legal obligation and its effects on the reserving state. These must be included at the time of signing or ratification, i.e. "a party cannot add a reservation after it has already joined a treaty".
Try to answer this question if possible (otherwise reply "unanswerable"): What are parties to a treaty forbidden to do after they have already joined a treaty?
add a reservation