No. If she chooses to identify with an immigrant culture in the U.S., she would do so as a South African-American.
Typically, cultural references point to the specific country and immigrant community with which one associates.
Since part of the importation of slaves involved the breaking process, in which the importers attempted to destroy as much of the cultural connection to their homelands as possible before selling them as slaves on this side of the Atlantic, and since few of the current nations of Africa actually existed at the time that the importation of slaves was curtailed, the descendants of slaves have chosen to employ a reference to a continent, rather than a reference to a country. Ms. Theron immigrated from a current nation state that happens to be located in Africa, so a reference to that nation state would be more appropriate.
Of course, then there will be some who cry that President Obama should be referred to as a Kenyan-American. Of course, for that to make sense, it would seem that he should actually associate with immigrants from Kenya. However, he married a woman whose ancestors were American slaves, lives/lived in a neighborhood heavily populated by the decscendants of slaves, attended a church with a congregation primarily composed of descendants of slaves, etc., and I am unaware of any actual associations he has with Kenyan immigrants, so the application of African-American to President Obama appears to be more strongly supported by his cultural associations, than not. Since I rarely employ the term, I figure those who do, (and those who go out of their way to take foolish offense), can fight that out among themselves.