Changing XP File Animations

Using Skinstudio, i am able to change most of the items, but what I can't change is the XP Progress animations.  I want to create my own animations, but there is no import button.  All it lists is a selection of the default progress animations that originally came with the skin.

Does anybody know how to change the progress animations in XP?  What program(s) can build such animations?  I would like to create my own, so it matches with the rest of my skin.