WordNet (r) 3.0 (2006):

sneak in
    v 1: enter surreptitiously; "He sneaked in under cover of
         darkness"; "In this essay, the author's personal feelings
         creep in" [syn: sneak in, creep in]
    2: insert casually; "She slipped in a reference to her own work"
       [syn: slip in, stick in, sneak in, insert]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "sneak in":
   affiliate, affiliate with, associate, barge in, break in,
   break in upon, burst in, butt in, charge in, combine, come between,
   come into, crash, crash in, crash the gates, creep in, crowd in,
   cut in, edge in, elbow in, encroach, enlist, enroll, enter,
   entrench, foist in, get into, go into, horn in, impinge, impose,
   impose on, impose upon, infiltrate, infringe, insinuate, interfere,
   interlope, interpose, intervene, intrude, invade, irrupt, join,
   join up, league with, obtrude, press in, push in, put on, put upon,
   rush in, sign on, sign up, slink in, slip in, smash in, squeeze in,
   steal in, storm in, take out membership, take up membership,
   team up with, team with, throng in, thrust in, trench, trespass,
   work in, worm in