You can't launch a nib. They are not executable.
I assume you mean something along the lines of: can I load an NSWindow that is serialised in another Nib and close the current key window? If so not is a single line of code. Assuming you are doing things correctly and each Nib has a single Window in it and a NSWindowController subclass as it's owner then you can simply create an instance of that NSWindowController subclass with the correct nib name and ask it to display it's window.
Note doing this in an action (something like -(IBAction) doSomething
id) sender) could be a bad idea unless you have some safe, sensible way to clean up that instance once you are done with it.
If you gave us some more concrete code to work from we might be able to give you some advice.