Add element to GUI

Hi,

I was wondering if it would be possible to add new elements (eg button) to an app GUI (eg iTunes). I know how to do a script and make it available in scripts menu, but, is it possible to add a new button to the iTunes GUI, and fire the script when the user press this button?

I tried searching UI Scripting associate with some kind of skin/theme modification, but found nothing. I'm not restricted to scripting. Any tip/hint in any language/technology (preferably those compatible with OS X 10.8.x) is welcome.

TIA,

Bob