The Collaborative International Dictionary of English v.0.48:

Diphthongize \Diph"thong*ize\, v. t. & i.
   To change into a diphthong, as by affixing another vowel to a
   simple vowel. "The diphthongized long vowels." --H. Sweet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diphthongize
    v 1: change from a simple vowel to a diphthong; "This vowel
         diphthongized in Germanic" [syn: diphthongize,
         diphthongise]