You can not read/write files from within a web page using JavaScript! So even you write a converter from SVG to DWG (isn't DWG a proprietary format?) you can not save it (you may send it to some server to do so, but again you may send SVG and convert it on the server)...
However you can use real-word units (like mm) in SVG - read here:
http://www.w3.org/TR/SVG2/coords.html[
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