If you are buying a RTW product (OWE or GE) then the JNB-ICN is included in the price as the rest of the ticket (though the mileage based fares might be severely stressed). For the OWE product you'd have to do it eastbound, flying ICN-HKG-JFK-LHR-JNB-HKG-ICN for example (adding the rest of the 20 segments as appropriate). You cannot do it westbound due to the restriction on entering Europe a 2nd time (i.e. LHR-JNB-LHR is prohibited on OWE tickets).