WordNet (r) 3.0 (2006):

add to
    v 1: have an increased effect; "This adds to my worries"

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "add to":
   aggrandize, amplify, augment, bloat, blow up, boost, broaden,
   build, build up, bulk, bulk out, crescendo, develop, dilate,
   distend, enlarge, exalt, expand, extend, fatten, fill out, fortify,
   hike, hike up, huff, increase, inflate, jack up, jump up, lengthen,
   magnify, maximize, parlay, puff, puff up, pump, pump up, put up,
   pyramid, raise, rarefy, recruit, reinforce, strengthen, stretch,
   sufflate, supplement, swell, thicken, up, widen