You can do it by phone. Just have your ticket number handy if you use other than QF. QF can be reached at 1 800 227 4500. QF, BA and AA all have ticket counters at SFO, but if you are just adding AA/BA flights your ticket can remain an e-ticket and you are much better off doing the changes by phone, it will take an hour in person. The reissue fee is paid once for a set of changes (so do them all at the same time). QF and BA do charge additional fees outside of the US; not sure if they do in the US. If there is a fee above the USD 125, you might want to use AA instead ... they can do the reissue, albeit with considerable difficulty as the QF computer system is completely incompatible with AA (at LAX they have to use separate computer terminals, it is that bad), and AA won't be keen to help you unless you flew AA trans-Atlantic or trans-Pacific.