Auto-create mockups from an existing app or image

I have a feeling this might be more complicated than it's worth, but I'd like your opinion anyway. Many of my projects involve making enhancements to existing applications or sites. Before I can design the UI enhancements, I need to create a mockup of the existing screen or web page, which can be a tedious process especially when the proposed change is minor from a UI perspective.

Would there be a way for the system to automatically create a mockup from a screenshot image? Or by pointing out the application itself maybe? As I'm writing it sounds insanely complicated, but I was thinking of applications like Quick Test Pro that can detect controls and objects on a screen.