How to get Sketchy shadows effect like pencil shading

It’s possible to do this in a post process but it will not look perfect since the space between those hatched lines will not have the same brightness as if unshadowed. This might get you started: http://abload.de/img/singlehathchingppk1r7r.jpg

If you are going for a fully stylized environment anyway your best bet is to do all lighting manually in it’s own post process, including the hatched shadows.
I just made a similar effect some days ago but with multiple hatched textures, http://abload.de/img/movieframe00383xcpb4.jpeg