First of all, I'd highly recommend rewriting your frames in Lua, and avoid using XML.
Secondly, to add a cooldown, you create a new frame with the "Cooldown" object type (instead of "Frame" or "Button"), parent and anchor to your frame, give it the same size as your frame (or use SetAllPoints, or SetPoint if you need offsets). Then :Show() your cooldown and use :SetCooldown(...) to start the sweeping animation.
See also:
*
http://www.wowpedia.org/Widget_API#Cooldown
*
http://www.wowpedia.org/API_Cooldown_SetCooldown (there's a code example here)