WordNet (r) 3.0 (2006):

tack on
    v 1: fix to; attach; "append a charm to the necklace" [syn:
         append, tag on, tack on, tack, hang on]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "tack on":
   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, subjoin, suffix, superadd, superpose, tag,
   tag on, unite with