I am unfortunately in the same position as you. Spent half the morning HUCA w/ Austrian, trying variations of the phrase ("I want to purchase an infant ticket") as they repeatedly told me they can't touch my existing reservation through Lifemiles. Then spent the rest of the day HUCA w/ Lifemiles/Avianca seeing if there was any way they could issue an infant ticket, which was unsuccessful. I guess I am gonna spend tomorrow morning HUCA w/ Austrian until I can reach a supervisor or something. Let me know if you end up getting this resolved, and I'll do the same if I figure it out before you.