First, I'd like to specify that "Khoisan" is sort of a hybrid term to describe two groups of people, the Khoi/Khoikhoi/Khoekhoe and the San/Saan/Basarwa. Second, I don't have any data that there is any sizable population of Khoikhoi or San people living in Zambia. The San seem to primarily live in Botswana, Namibia, South Africa, and Angola, while the Khoikhoi seem to live mostly in Namibia, Botswana, and South Africa alone. Third, it seems that when the Bantu southward migration began around 2000 BC (where a group of distinct peoples began migrating into Southern Africa along with new metallurgical technology and grazing animals), there were Khoi and San peoples already living there. There is evidence of much older groups of people living in these areas during the Upper Paleolithic, and presumably some of these people could have been ancestors of some modern Khoi and San individuals.