Do you need to validate the format of the XML or just Parse it for processing?
If you just need to parse it then the *obj/xml object will likely provide what need. It will parse the XML into a object node tree making for easy access from within PxPlus. It also performs a basic form check (valid tags, etc..) and can create new XML from a XML object node tree.
If you need to validate against the DTD we would suggest a 3rd party tools such as rxp or xmlvalid both of which claim to validate against the source DTD definition (if accessible)