The word "additory" is extremely rare and difficult to find synonyms for. The closest possible synonyms could be "additive", "augmentative" or "complementary". These words imply something that adds value or enhances something else, in the same way that "additory" implies something that is able to add or contribute to something else. "Addendum" or "supplementary" may also be used as synonyms in certain contexts, particularly when referring to additional information or an addition made to a document or piece of work. Overall, while few suitable synonyms for "additory" exist, there are other similar words that can effectively convey the same meaning in different contexts.