The Collaborative International Dictionary of English v.0.48:

Imperialize \Im*pe"ri*al*ize\, v. t. [imp. & p. p.
   Imperialized; p. pr. & vb. n. Imperializing.]
   To invest with imperial authority, character, or style; to
   bring to the form of an empire. --Fuller.
   [1913 Webster]