The standard way to affect what parser is used is to specify a system property and its value. If you want to have a particular DOM parser used, then you find the name of the DocumentBuilderFactory class that creates that parser (fully qualified class name of course) and assign its name to the javax.xml.parsers.DocumentBuilderFactory system property. Or if it's a SAX parser you want used, then you find the name of your SAXParserFactory and assign that name to the javax.xml.parsers.SAXParserFactory system property.
Of course it's possible your package might prevent this from working by hard-coding the Xerces parser class into their code. In that case there is nothing you can do except to get the code changed.