Import current UI into sketchflow

I need prototype the UI for next gen of my ASP.Net app. I'd like to use a tool like Sketchflow to mock of the new screens and updates to the currents screens.

Can I import my current UI into Sketchflow? Is there another prototyping tool that does allow importing existing UIs?

Screen shots are good (this is how Sketch Flow imports PowerPoint presentations), but I guarantee your audience will focus too much on the current look if it is not "Sketchy".

A good trick for existing layouts is to cut/paste* an image/screen-shot into a Sketchflow page, then trace over it using standard sketchflow rectanges, buttons, pencil tool etc.

When you are done, remove (or simply hide) the reference image. The end result has the rough layout and functionality, but looks like a prototype.

*Tip: You can paste an image straight from the clipboard into Expression Blend pages if you use a paint package (Paint.Net is great for this). The Pencil sketch effect option in Paint.Net (below) also helps convert existing stuff into a more Sketchy look.