The Collaborative International Dictionary of English v.0.48:

Superscribe \Su`per*scribe"\, v. t. [imp. & p. p.
   Superscribed; p. pr. & vb. n. Superscribing.] [L.
   superscribere, superscriptum; super over + scribere to write.
   See Super-, and Scribe.]
   To write or engrave (a name, address, inscription, or the
   like) on the top or surface; to write a name, address, or the
   like, on the outside or cover of (anything); as, to
   superscribe a letter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superscribe
    v 1: write on the top or outside; "superscribe one's name and
         address"
    2: write on the outside or upper part of; "superscribe an
       envelope"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "superscribe":
   address, copy, copy out, direct, draft, draw up, edit, enface,
   engross, inscribe, make a recension, make out, pen, pencil,
   push the pen, put in writing, recense, record, revise, rewrite,
   scribe, scrive, scroll, spill ink, spoil paper, trace, transcribe,
   type, write, write down, write out