I've cut a number of pieces of custom straight and curved crown molding with Aspire. As you have noted, Aspire's only shape for starting blank is a cube. However, that has never been much of a hindrance. If I want to create a "blank" as a starting place, I just create a few vectors and fake toolpaths for the blank. If you preview those first, then double click to remove the waste material, then you can more easily see the preview of the toolpaths actually used to cut the profile.
For what it's worth, the right toolpath for this is the Moulding path. I have made these using 3d finishing toolpaths, before the moulding path was available in the software, and had problems both with the speed and finish quality. The moulding toolpath solved these issues.
D&S Artistic Woodworking http://www.dsartisticwood.com