If you also render out a sketch layer of a suitable type (you may need to experiment for a while, on that) and the same size as your toon render, then in Photoshop you can use a big soft eraser to remove most of that sketch layer, just leaving shading that looks sort-of like hatching (see picture, below). Of course you have to run an Action to knock out all the white first, leaving just the sketch lines laid over your toon lines and colour flats layers.

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Learn the Secrets of Poser 11 and Line-art Filters.