The recent thread on schwa got me to thinking about epenthesis 'the insertion of a sound or letter into the body of a word' (M-W) or 'the insertion of a sound in the middle of a word, as in Middle English thunder from Old English thunor' (A-H). If the insertion takes place at the beginning of the word, as especial and estate, it's called prothesis. If it's a vowel rather than a consonant that's being epenthetically inserted, it can be called anaptyxis.