The Collaborative International Dictionary of English v.0.48:

Herewith \Here*with"\, adv.
   With this.
   [1913 Webster]

WordNet (r) 3.0 (2006):

herewith
    adv 1: (formal) by means of this; "I hereby declare you man and
           wife" [syn: hereby, herewith]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "herewith":
   arm in arm, by, by dint of, by means of, by use of, by virtue of,
   by way of, cheek by jowl, hand in glove, hand in hand, hereby,
   in virtue of, on, per, shoulder to shoulder, thanks to, thereby,
   therewith, therewithal, through, upon, whereby, wherewith,
   wherewithal, with