No visa, but how long is your connection? You may have to clear immigration and pick up any checked luggage, then re-check-in with Air Namibia. The lineups to get through JNB immigration between 6a and 10a can take over an hour. Ethiopian may not check your bag through on separate tickets. I don't recall seeing an airside transit at JNB and all passengers are channeled to the immigration/customs hall then landside.