Hmmm... I dont beleive there is a way to do this (at least with apple's software). According to machelp (which is hardly ever helpful), it says "You must type the command exactly as it appears in the application menu". And since iTunes doesn't have the play or pause button in any of the menu bars, there is no way to give it a shortcut.
Search "global keyboard shortcuts" in mac help to see what I am talking about. Seems really odd to me that they wont let you customize the play/pause button.