WordNet (r) 3.0 (2006):

throw in
    v 1: add as an extra or as a gratuity
    2: give up in the face of defeat of lacking hope; admit defeat;
       "In the second round, the challenger gave up" [syn: drop
       out, give up, fall by the wayside, drop by the
       wayside, throw in, throw in the towel, quit, chuck up
       the sponge] [ant: enter, participate]
    3: to insert between other elements; "She interjected clever
       remarks" [syn: interject, come in, interpose, put in,
       throw in, inject]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "throw in":
   contribute, drag in, ease in, edge in, fill in, foist in, fudge in,
   implant in, infuse, inject, inject in, inoculate, insert,
   insert in, insinuate, insinuate in, intercalate, interjaculate,
   interject, interpolate, interpose, intervene, introduce,
   introduce in, intromit, intrude, lug in, perfuse, pop in,
   put between, put in, run in, sandwich, set in, slip in, smuggle in,
   squeeze in, stick in, thrust in, tuck in, wedge in, whip in,
   work in, worm in