diphthongize

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive & intransitive verb To pronounce as or become a diphthong.

from The Century Dictionary.

• To change, as a vowel, into a diphthong: thus the u of many Anglo-Saxon words has been diphthongized into ow in modern English, as in the word now.
• To unite in forming a diphthong.
• Also spelled diphthongise.

from the GNU version of the Collaborative International Dictionary of English.

• verb To change into a diphthong, as by affixing another vowel to a simple vowel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive, linguistics To change to a diphthong (as by inserting or removing a vowel).
• verb intransitive, linguistics To become a diphthong.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb change from a simple vowel to a diphthong