Add a new UIViewController to the storyboard. Remove its main view so that the view controller is empty. Set the Class of the view controller (in the Identity Inspector) to that of your nib's File's Owner. Also make sure the .h and .m files are added to your project already.
Now open the nib, select the top-most view and press Cmd-C to copy it to the clipboard. Switch to the storyboard, select that new, empty view controller. Press Cmd-V to paste the contents of the nib into this view controller.
You may have to reconnect all the outlets and actions, but at least you don't have to do the layout again. :-)