The rules ex-Asia, ex-South West Pacific and ex-North America specifically state that you can buy extra segments in a lower class than the RTW ticket. I can't imagine that the rules would be different ex-Europe. I had a partially used 1st class ticket ex-Africa reissued by AA in Australia 2 weeks ago and added 4 extra segments in South America in economy class and AA just went straight ahead and charged the Y class $US100 per segment - there seemed to be no uncertainty on their part.