I made an iPhone app long ago and I'm now thinking of making it a universal app, but I was wondering if I could use Storyboards for making the iPad version, so for the iPhone it would load a MainWindow.xib and for the iPad it would load a MainStoryboard file. Is this possible?
Yes, but you actually have to make two separate storyboards (1 for iPhone and 1 for iPad).
In the iPhone storyboard, just drag your mainwindow.xib into the middle of the rootViewController and make sure it goes full-screen.
Then, you can do whatever you want with your iPad storyboard.
Make sure that your two storyboard files are linked correctly in your project summary though!