Door frame's too short to put up a Pull Up bar, help?

Hi, my door frame is too thin to put up a pull up bar, what are the extensions called to make your door frame wider? this guy used it in his video but he didnt make it clear what it was called..

skip to 1:30
http://www.youtube.com/watch?v=ZYwvDOLSU10&feature=related
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