I did this in February, converting a CX-issued, HKG-originating 3 continent ticket to a 4-continent ticket (added S. America) by contacting AA's RTW desk-- just paid an add/collect for the difference.
This was done after I flew from HKG to North America.
[This message has been edited by HKG_Flyer1 (edited 06-24-2001).]