I doubt you could. Nyasaland (Malawi today) was simply not gonna be controlled by the British. Independence was a matter of when, not if.
Rhodesia is easier, but still hard. You'd have to get a much larger white population or a much better educated and more loyal black population to keep it in the Commonwealth. You could unite the two Rhodesias, but that would just be Rhodesia, not the Central African Federation. Perhaps they'd have to gain a territory or two to keep that name.