I had the agent at T5 GC South do my most recent complex itinerary booking. It took more than a few minutes, but she managed it in the end, due partly to very thin availability on some segments of the itinerary. If you're on the way through anyway, this is a reasonable way to do it but make sure you have enough time. You will also, at least for any travel to the USA, need to supply passport details for all travellers to get the ticket issued so make sure you have that available as needed.