You can't buy a RTW that is partially based on a business-class fare and partially based on an economy-class fare.

However, you can of course buy a business-class RTW and choose to sit in Y. Or, you can buy a business-class RTW in which the trans-Pacific portion is BKK-JFK (TG), SIN-EWR (SQ), or HKG-YYZ (AC). On these flights, there is a surcharge to sit in business class, but no surcharge to sit in economy. On the TG flight, the seat woulkd be in premium economy, which is an enormous improvement over any regular economy (incl E+).