The Collaborative International Dictionary of English v.0.48:

Interpolate \In*ter"po*late\, v. t. [imp. & p. p.
   Interpolated; p. pr. & vb. n. Interpolating.] [L.
   interpolatus, p. p. of interpolare to form anew, to
   interpolate, fr. interpolus, interpolis, falsified, vamped
   up, polished up; inter between + polire to polish. See
   Polish, v. t.]
   [1913 Webster]
   1. To renew; to carry on with intermission. [Obs.]
      [1913 Webster]

            Motion . . . partly continued and unintermitted, . .
            . partly interpolated and interrupted. --Sir M.
                                                  Hale.
      [1913 Webster]

   2. To alter or corrupt by the insertion of new or foreign
      matter; especially, to change, as a book or text, by the
      insertion of matter that is new, or foreign to the purpose
      of the author.
      [1913 Webster]

            How strangely Ignatius is mangled and interpolated,
            you may see by the vast difference of all copies and
            editions.                             --Bp. Barlow.
      [1913 Webster]

            The Athenians were put in possession of Salamis by
            another law, which was cited by Solon, or, as some
            think, interpolated by him for that purpose. --Pope.
      [1913 Webster]

   3. (Math.) To fill up intermediate terms of, as of a series,
      according to the law of the series; to introduce, as a
      number or quantity, in a partial series, according to the
      law of that part of the series; to estimate a value at a
      point intermediate between points of knwon value. Compare
      extrapolate.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

interpolate
    v 1: estimate the value of [syn: interpolate, extrapolate]
    2: insert words into texts, often falsifying it thereby [syn:
       interpolate, alter, falsify]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "interpolate":
   add, admit, annex, append, drag in, edge in, enter, fill in,
   foist in, fudge in, implant in, inject in, insert, insert in,
   insinuate, insinuate in, intercalate, interjaculate, interject,
   interlope, interpose, intervene, introduce in, intrude, lug in,
   put between, run in, sandwich, smuggle in, squeeze in, superadd,
   throw in, thrust in, wedge in, work in, worm in