The Collaborative International Dictionary of English v.0.48:

Symmetrize \Sym"me*trize\, v. t. [imp. & p. p. Symmetrized; p.
   pr. & vb. n. Symmetrizing.] [Cf. F. sym['e]triser.]
   To make proportional in its parts; to reduce to symmetry.
   --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

symmetrize
    v 1: make symmetric; "symmetrized waves" [syn: symmetrize,
         symmetrise]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "symmetrize":
   assimilate to, balance, coordinate, damp, equalize, equilibrize,
   even, even up, flatten, harmonize, homogenize, integrate, level,
   make uniform, normalize, proportion, proportionate, regularize,
   regulate, smooth, stabilize, standardize, stereotype, uniformize