The Collaborative International Dictionary of English v.0.48:

Superadd \Su`per*add"\, v. t. [imp. & p. p. Superadded; p. pr.
   & vb. n. Superadding.] [L. superaddere. See Super-, and
   Add.]
   To add over and above; to add to what has been added; to
   annex, as something extrinsic.
   [1913 Webster]

         The strength of any living creature, in those external
         motion, is something distinct from, and superadded
         unto, its natural gravity.               --Bp. Wilkins.
   [1913 Webster]

         The peacock laid it extremely to heart that he had not
         the nightingale's voice superadded to the beauty of his
         plumes.                                  --L'Estrange.
   [1913 Webster]