Not sure what you're asking, but I'll take a guess:
You want to use another XIB's view inside a XIB.
In that case, grab a view in the Library, and drag it into the original view. It'll show up in IB's Document window (look for it in the Window menu). Then, double-click the view in the Document window, and edit it as necessary.
Check out
Lecture 3 & 5 of CS193P for more info.