But of course if he is already in HKG and wants to go to NY via Sydney, it will mean that he intends to add one extra continent to his RTW ticket (if that is what he has).

Yes it will be possible - if you pay for the extra fare for the addition of the extra continent you are travelling. Then you would need to get your ticket reissued and fare recalculated.

If you're traveling from HK to NY in Biz, your fare will be recalculated as Biz from HK to SYD to NY.