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):

subjoin
    v 1: add to the end

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "subjoin":
   add, adjoin, affix, agglutinate, annex, append, attach, burden,
   complicate, conjoin, decorate, encumber, glue on, hitch on, infix,
   join with, ornament, paste on, plus, postfix, prefix, put with,
   saddle with, slap on, suffix, superadd, superpose, tack on, tag,
   tag on, take, unite, unite with