NSWindow does not have scrolls on it, rather that is a NSScrollView inside it. You could go in and use the setHasVerticalScroller: and setHasHorizontalScroller: methods. I have not tried that, but it looks like that would do what you want.
However, I am going to say that this is probably a really bad idea for almost all uses. Not everyone has a scroll wheel (or knows how to use it... I am not kidding here). And it makes your app really non-discoverable and breaks the typical usage patterns. So in general a bad idea.