In short...you can't.
There's a workaroud I use to create the iPad storyboard at the end of the iPhone development.
You have to locate the iPhone story board and using the Terminal you can duplicate it giving it the name of the iPad storyboard that Xcode has created for you. Delete the empty old storyboard and open the newly copied one changinge the targetRuntime attribute to "iOS.CocoaTouch.iPad" instead of the old value "iOS.CocoaTouch".
Open the iPad storyboard with Xcode, everything is there but it still needs to be arranged and connected to the ViewController (wiring stuff included..). It's not perfect but I usually do it to save some error prone copy and paste (and several hours as well).
If you just want to target your app for iPhone and deliver it to iPad you can just use the iPhone Xib and you'll get the 2x little botton in your right corner.
Update late 2014 (after Xcode 6 beta release)
Xcode 6 now allows you to define a single storyboard and through the size classes concept, to make it adapt to all their devices. This solution is a bit confusing (especially if you're familiar with how this problem is solved in the Android environment) but it's something in the right direction IMHO.