Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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