First of all, don't replace any headers which are provided by Apple.
Generally, it's done one of two ways:
1) You can copy some of these headers to your project and just include these files the same way as you include any other headers
#import "SomeHeader.h"
2) Sometimes you have to sanitize them (edit them) a little bit. Quite often, these headers has something like in in them:
#import "NSObject.h"
And compliller won't be able to find it, because NSObject is built-in class. So, you need to remove this like.
3) If you just need couple of methods out of it, then Tuukka Nori solution is right.
On top of these, you will need to link (statically or dynamically) against appropriate private framework (just including headers isn't enough).