Hi Stephane I also have South African Ancestry and have just found this surname in my tree. My Grandfather: Joseph Francis Van Rooyen had a brother John George Marcellin,all the other brothers had family surnames as middle names so I'm assuming this brother also did.
My great uncle Stephen had also told me a long time ago that he had french ancestry.I believe the marcellin line would have come through their mother Ellen Maria oliver/olivier.
A search of the south african archives only produces one marcellin born before 1955 and even then it is also a middle name.
DE LAGE, JULES LEON MARCELLIN.
The only other marcellin IS : IMMIGRATION PAPERS - MISS J MARCELLIN.1955
and a divorce in 1977 of: DIVORCE. MARILYN ELIZABETH MARCELLIN (BORN IRWIN)VERSUS GERARD HENRI MARCELLIN.
This line of my family also came from the eastern cape/natal.