aldoni

Aldoni is al plus doni, and it keeps the shape of both: the thing you add is the object, and where it goes takes al. So aldoni akvon al la vino, never the other way round. It works for a remark tacked onto a conversation just as well as for an ingredient, which is why speakers so often end a paragraph with mi nur aldonu, ke.... The noun aldono covers an appendix, a supplement or an addition, and aldona means additional. Arithmetic prefers plus or adicii, but nobody will misunderstand aldoni there either.