There are several ways to go about that, Arcadia. Here are a few. :)

If the hair is a mesh object, you could use morphs or, as Rockadon pointed out, use transmaps.

 All you have to do to make a basic transmap is load the template into your photo editor, make the hair you want visible solid white(maybe with blurring and smudging along the edge so it looks like realistic) and the background black.

I hope it helps!