As has been suggested, it is probably less than optimal to do it, but there are commercial options for doing just that. The Lens Baby lenses (at least the 3G, which I have) uses a set of aperture rings that are held to the front of the lens by magnets. Also, Cokin makes a holder for cards that come with preshaped non-round apertures for making out of focus points of light come out the shape you want. You can also cut your own for this holder. There are probably other options, but these are the two I have, or rather had. I have the Lens Baby 3G, but I lost the rings, so it is stuck at f/2.