Yes, no problem adding sectors after the first flight, in fact used to be quite common to issue skeleton AONEx initially and fill in the sectors later, eg. LHR-LAX-SYD-LHR for the initial ticket, to help tardy TAs or ticket desks. Now it is much less hassle to get it right the first time, plus the fees have gone up. Most airlines will charge a small convenience fee for using their airport ticket desk, or phoning, etc. so it will be maybe an extra $50 as well (depends on which airline does the reissue) but quite viable and a good solution to your problem. Though most CC will do an emergency credit limit increase to accomodate a single airline ticket purchase (I've had to go $20K over my limit in the past and it was approved in seconds, but this depends on your issuer).