i would take a cube and modell the key step by step, extrude, subdivide and so on. with the add tool i wouldn't work, to inconvienent for this, i think. make the key with different objects, non one, e.g. cylinder for shaft... .
-create primitives that fit as close as possible your rotoscoped image, -then create points with snap (to points) activated, place those points on the points of the primitives, -when you're done, hide or delete the 3d templates (the primitives you created), -create polygons between your points with the Add Polygon tool.
You can do this a little at a time, like creating a bunch of points and adding polygons, then creating another bunch of points, and so on.