if you have Plus, you can define your shortcode by putting the PHP in Advanced options > Head Section > Action & Filter Box.
if not, then you will need to create a Child theme and make your modifications in there. Below is an example I did in a tutorial. But I am no PHP expert so I cant help with any of the details if you go the PHP route
http://forum.weavertheme.com/discussion/8406/actions-filters-tips-and-examples-part2