While SH3D will export to SVG 2D drawing, it is not possible to import SVG directly. One workaround is to convert the SVG to a raster image (Inkscape/theGimp - or a full-screen browser screenshot) and use as background image.

PS - depending on the SVG file, you may be able to rip apart the co-ordinates of the SVG and transfer more or less manually to the home.xml file - see for instance thread 7674.