Hi,

the easiest method imo would be to import the shape in Photoshop, make it white in there with a black background and apply several times a gaussian blurr with setting from big to small. After that bring the image into ZSurf (free) and export it as .iges file. In MoI (depending on your Photoshop experiments) you would now have a nicely rounded shape. The last step would be then to trim the shape from the bottom square, with a line.

Hope this helps.

Regards
Stefan