You are on the right track. It certainly is allowed but will require calling AS. You can change, but not add to your award online. I usually throw an extra segment onto my awards which I can change later. I can build it into a stopover (if I am not already using a stopover) or even change the destination, all online. The change fee waiver is "golden." Since you are requiring flights outside the booking window, this is not an option for you.

Don't forget that you can add a stopover in NAN if you like too. Perhaps a couple of days on the beach before BNE. Keep in mind though that NAN has one of the most expensive departure taxes in the Pacific. You are, however, not required to pay it if your onward flight is less than 72 hours so best to keep stopovers less than 3 days.

A friend recently booked 2 EK J awards when AS was having issues and a AS flight couldn't be added at booking. Once those were resolved we were able to call AS and add 2 LAX-HNL Saver A flights to the award.

FYI: You can also change awards that require repricing. It will be the additional miles plus fees and taxes. No worries about keeping the existing award intact. I booked CX J MEL-HKG. (30K) A month later I called and added CX F HKG-LAX with a 2 month HKG stopover. (80K) AS collected an additional 50K and a small difference in fees & taxes.

I also recently changed an award on QF starting in BNE to CX starting in PER while maintaining my AS F LAX-FLL flight after an LAX stopover.

James