add-on

(noun) a component that is added to something to improve it. Synonyms: addition, improver. Examples: "The addition of a bathroom was a major improvement." "The addition of cinnamon improved the flavor."

(noun) a supplementary component that improves capability. Synonyms: accessory, appurtenance, supplement.

(verb) make an addition. Synonyms: add on. Example: "Let's add on to this."

(verb) add to the very end. Synonyms: append, add on, supplement, affix. Example: "He appended a glossary to his novel where he used an invented language."