The Collaborative International Dictionary of English v.0.48:

Additament \Ad*dit"a*ment\ ([a^]d*d[i^]t"[.a]*ment), n. [L.
   additamentum, fr. additus, p. p. of addere to add.]
   An addition, or a thing added. --Fuller.
   [1913 Webster]

         My persuasion that the latter verses of the chapter
         were an additament of a later age.       --Coleridge.
   [1913 Webster]