Roberto,

There are two ways how you can access XML that does not have a getXXX() accessor:

 (1) Grab the DOM node and navigate using the DOM API:
   Node node = xmlobject.getDomNode();
   NodeList ch = node.getChildNodes();
   ...etc.

 (2) Use built-in support for XPath.  The XmlBeans API uses a dialect of XPath, please consult http://xmlbeans.apache.org/ for further details. 
Below is a sample code that reads xsd from the attached file:

        XSSFWorkbook wb = new XSSFWorkbook("CustomXMLMappings.xlsx");
        List<POIXMLDocumentPart> rel = wb.getRelations();
        for(POIXMLDocumentPart p : rel){
            if("http://schemas.openxmlformats.org/officeDocument/2006/relationships/xmlMaps".
                    equals(p.getPackageRelationship().getRelationshipType())){


                InputStream is = p.getPackagePart().getInputStream();
                CTSchema ct = CTSchema.Factory.parse(is);

                //select XML tree for the  schema
                String query =
                    "declare namespace w='http://schemas.openxmlformats.org/spreadsheetml/2006/main';" +
                    "$this/w:MapInfo/w:Schema";

                XmlObject[] o = ct.selectPath(query);
                for (XmlObject xsd : o){
                    XmlCursor cur = xsd.newCursor();
                    cur.selectPath("./*/*");
                    while (cur.toNextSelection()) {
                        XmlObject ch = cur.getObject();
                        System.out.println(ch);
                    }
                }
            }
        }

Regards,
Yegor