In terms of continents, your original itinerary is an xONE5 comprising:

Africa --> Asia --> Sth West Pacific --> Sth America --> Europe --> Africa

If you want to now go to Nth America, you can do this by:

• changing the ticket to an xONE6 and paying the extra cost, or
• dropping one of the other continents in order to add Nth Am and paying the change fee (US$125)

Whatever changes you make, the whole end-to-end trip has to still be a valid xONEx ticket - in your case in particular that it ends up back in Africa.

Note than you cannot drop Europe - because to get from the Americas back to Africa you must go via Europe. Since you intend to keep Asia, your choice is to drop either SWP or Sth Am (or both - but there is no refund for dropping to an xONE4).

After departure you can cancel the remaining sectors of the ticket - paying the cancellation fee and obtaining a refund of the remainder (if any) - see rule 16(b) of the rule sheet

Of course, since you haven't yet started, you can cancel the whole trip, paying the cancellation fee and getting the remainder refunded.