RTW question

I can't get a straight answer out of VS..go figure..ask one person get one answer and so on...
Anyways...I understand that you can only head east or west..we are heading east LAX/SFO - LHR..but I was thinking of throwing in a LHR - JNB or CPT flight for giggles..and can't get an answer if a round trip is allowed as this is South-North..then continue on our merry way east.
Anyone have a clue..or when it comes time to book it should I just hunt for an agent that agrees to book it [:I]
Anyways...I understand that you can only head east or west..we are heading east LAX/SFO - LHR..but I was thinking of throwing in a LHR - JNB or CPT flight for giggles..and can't get an answer if a round trip is allowed as this is South-North..then continue on our merry way east.
Anyone have a clue..or when it comes time to book it should I just hunt for an agent that agrees to book it [:I]