I had the same question put to the farmer we hunted with and his reply was as follow:
A small breeding herd was bought by the Johannesburg Zoo but it was quickly determined that the stress of being in captivity was having a negative influence on their health. A local farmer then made a deal with the Zoo during 1985 or 1986 (I can't remember exactly the year) and swapped 5 Black Wildebeest for four Barbary Ewes and one Ram. The 5 Sheep were placed in a 3m high boma when they arrived on the farmers property and low and behold, by the next morning all 5 had escaped from the boma by jumping out of their enclosure. The Barbary's were then not seen for quite some time but they managed to adapt to their new surroundings quite well and started breeding, and the rest, as they say, is history.
There are however numerous farmers in the Northern Cape that protect these animals on their land and will not allow anyone to hunt them at all. One particular farmer has a well over a 100 on his property at any one given time.
Please bear in mind that this is the story that was told to me as how they were introduced to the Northern Cape - I am quite sure that there will be different versions of how they were introduced into that particular area.
Regards
Boela.