The speakable items are just Applescripts. Open one up, see how it constructed, and then proceed to modify it.
I'm not sure if iTunes is scriptable, so that might be a problem...
Another (albet costly) solution : Via Voice for OS X... It ships this month and it should be able to do that. From what I understand, you can use it in various applications to control them via voice activiation. They showed the Finder and Mail both being run by voice activation at MWNY and it was pretty impressive.