Start a sketch tools>sketch tools>insert picture. I recommend washing out the image with brightness/contrast adjsutments before you insert it - nothing worse than editing a black line on a black image, eh?
FYI - There is also a free raster to vector utility called Wintopo which is a really fast way to get a crummy sketch (disjoint lines, segmented curves, etc). But , in some cases, that might do.
-Ed