No. The journey must originate from Africa to be eligible for the Africa baggage allowance. For example: Flights from Johannesburg to Doha where the first flight on the booking departs from Africa will be eligible for the allowance and not the return sector.
Please check the link below for the standard checked baggage allowance and dimensions
To purchase excess baggage login to Manage Booking (you will require PNR/ticket number/Privilege Club number and passenger's last name).
Once you retrieve your booking, then click on "Purchase Excess baggage" on the right panel to purchase excess baggage online.