code: public XSDocument addImport(String namespace, String schemaLocation) throws SAXException{
          xml.startElement(URI_XSD, "import");
          xml.addAttribute("namespace", namespace);
          xml.addAttribute("schemaLocation", schemaLocation);
          xml.endElement();
          return this;
      }
docs: <xsd:import [namespace="$namespace"] [schemaLocation="$schemaLocation"]/>