The Collaborative International Dictionary of English v.0.48:

Subjoin \Sub*join"\, v. t. [imp. & p. p. Subjoined; p. pr. &
   vb. n. Subjoining.] [Cf. OF. subjoindre, L. subjungere. See
   Sub-, and Join, and cf. Subjective.]
   To add after something else has been said or written; to
   ANNEX; as, to subjoin an argument or reason.
   [1913 Webster]

   Syn: To add; annex; join; unite.
        [1913 Webster]

WordNet (r) 3.0 (2006):

subjoining
    n 1: the act of supplementing [syn: supplementation,
         subjunction, subjoining]