The Collaborative International Dictionary of English v.0.48:

Supererogate \Su`per*er"o*gate\, v. i. [imp. & p. p.
   Supererogated; p. pr. & vb. n. Supererogating.] [L.
   supererogatus, p. p. of supererogare to spend or pay out over
   and above; super over, above + erogare to expend or pay out
   money from the public treasury after asking the consent of
   the people. See Super-, and Erogate, Rogation.]
   To do more than duty requires; to perform works of
   supererogation; to atone (for a dificiency in another) by
   means of a surplus action or quality.
   [1913 Webster]

         The fervency of one man in prayer can not supererogate
         for the coldness of another.             --Milton.
   [1913 Webster]