Screen shots are good (this is how SKetchflow 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.
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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.
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*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.
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