WordNet (r) 3.0 (2006):

slip in
    v 1: 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:

94 Moby Thesaurus words for "slip in":
   admit, barge in, be admitted, break in, break in upon, breeze in,
   burst in, bust in, butt in, charge in, come barging in,
   come between, come breezing in, come busting in, come in, crash,
   crash in, crash the gates, creep in, cross the threshold, crowd in,
   cut in, drop in, ease in, edge in, elbow in, encroach, enter,
   entrench, foist in, gain admittance, get in, go in, go into,
   have an entree, have an in, hop in, horn in, impinge, impose,
   impose on, impose upon, infiltrate, infringe, infuse, inject,
   inoculate, insert, insinuate, interfere, interject, interlope,
   interpose, intervene, introduce, intromit, intrude, invade, irrupt,
   jam in, jump in, look in, obtrude, pack in, perfuse, pop in,
   press in, push in, put in, put on, put upon, rush in, set foot in,
   set in, slink in, smash in, sneak in, squeeze in, steal in,
   step in, stick in, storm in, take in, throng in, throw in,
   thrust in, trench, trespass, tuck in, visit, wedge in, whip in,
   work in, worm in