Of course you can!! Nancy has always been depicted as white. She was intended to be white. It is extremely unlikely that she would be a black character.
You can spoil the reality of a book which is filmed with characters substantially different from those written. Jane Austin's characters are white. If non white actors are cast for some characters it would not work from a historical class and social point of view. You could cast one of Austin's characters with a white actor speaking with a working class London accent - that wouldn't work either.
A recent Poirot episode cast a black actress as a character's mistress. This was unreal - since the original character was white - and had she been black, by the mores of the time, she would have been shunned by most and there would have been much whispering behind her back. For me, this casting was therefore absurd. (Although the actress herself was excellent in the part).
Cross ethnic casting can work better in classic stage plays, since they are more styalised less realistic than films or TV dramas, and people will accept a black actor playing Hamlet, for example.
Othello is another case in point. In the play he is a Moor - an Arab man. But he has been depicted on stage and film as a black man.