It surprises me that although nga occurred 119 times in my recorded corpus of 1962, nothing is said about it in Samarin (1967). This word is being used with other functions in contemporary urban Sango, introduced, I think, by speakers of Ngbandi, in whose language it means more than 'also.' It is my impression that nga occurs more frequently in a verb phrase than in a noun phrase.
Source: Samarin 1967: 237
Source: Keesing 1988