Hi,
There are many things in SVG that Dia does not support, but a rough importer would indeed be nice, and shouldn't be impossible -- the XFig importer could be a good example to work from.
There is a rough SVG importer already. So the basic workflow SVG-> Open as Dia Diagram -> Dia .shape is possible. I just created a little piece of SVG in inkscape and turned into a Dia shape. Regards, Steffen