Hi. I'm also a sort of newbie to SH3D, but I really can't understand your problem.
As Hans explained in the other post of yours, you can export 2d plans to SVG. These seem decent enough, from my trials (Doors have, perhaps, an extra line, which you can delete in another program.)
Many programs allow you to work in SVG - including Dia (mentioned in the site you linked to.) From your link, I assume you're working in Linux (?). Some Multi-OS programs are Dia, LibreCad and Inkscape (not sure if this last will allow you to draw in scale, but the others should.) And, I'm sure, there are many more free CAD programs in Linux, ow whichever OS you want.
HTH,
maker