Welcome to AH and a great site. Just a note: remember that black wildebeest, blesbuck and the common impala are exotics in Namibia. They are native to South Africa...
Werner, What you have said is somewhat correct...
The Impala is endemic to some parts of Namibia such as the Caprivi and part of the Kavango region. In South Africa, Impala are also endemic to only some regions, mostly Limpopo and Swaziland. This species has been reintroduced widely, both in private areas and nature reserves throughout most of South Africa and Namibia.
Black Wildebeest have been almost completely exterminated, the natural populations of this species is found in the Free State of South Africa, but the species has been reintroduced widely, both in private areas and nature reserves in South Africa and introduced throughout most of South Africa, Lesotho, Swaziland, Namibia.
Blesbok are endemic to South Africa, although natural populations of this species is found only in some regions such as North West, Free State, Eastern Cape, Gauteng, Kwazulu Natal and Mpumalanga. The species has been introduced widely to both private areas and nature reserves throughout most of South Africa and Namibia.
Obviously animals do not see boundaries or country limits as we have defined them, therefore if it is your goal to only hunt truly endemic species you would have to travel to at least a couple of region in South Africa or a region in South Africa and Namibia to accomplish this.
Personally if an animal has been introduced to an area of South Africa or Namibia and can be hunted there, there is no difference...