追加 (tsuika) means “extra” | MotiTan Japanese Dictionary

More examples

料理の追加は、店員にお伝えください。ryō ri no tsui ka wa ten in ni o tsuta e ku da sa i

Please tell the server if you would like to add another dish.

Presents an added amount or item that goes beyond the original amount or set.

追加注文tsuikachūmon: A further order made in addition to an earlier order.
おかわりokawari: A second serving of the same food or drink.

もとの数量や内容に、あとから加えること。また、あとから加えたもの。mo to no sū ryō ya nai yō ni a to ka ra kuwa e ru ko to ma ta a to ka ra kuwa e ta mo no

The noun 追加 (tsuika) combines 追 (tsui, "follow") with 加 (ka, "add"), so its form reflects adding something after an existing item.

追加注文 (tsuika chūmon) is an additional order in restaurant talk, whereas おかわり (okawari) usually refers to a refill or another serving of the same food or drink.