I have had an extensive debate with my TA on a 'RTW ticket including a UA sector (ex USA). The "official" word from UA is that they do not require middle names on the ticket "for the time being" even if there is a middle name in the Passport. Since the ticket was issued with my first and last name only (no middle name), the USA advance screeing process accepted my details which did include my middle name as per Passport.

I'm pretty sure you won't be able to change the ticketted name (ie adding a middle name) once ticketting has been done. You could ask that a notation be made on your PNR of your name as per Passport. This may help - but unsure.