External & Internal DTD

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I'm trying to add a child-element to an element declared in an external DTD.So far, I been able to add an attribut to an existing element but I can'tchange the definition of an externaly declared element. Can this be done?My External.DTD is :<!ELEMENT Root (Element1*)><!ELEMENT Element1(SubElement1*, SubElement2*)><!ELEMENT SubElement1(#PCDATA)><!ATTLIST SubElement1 PrimaryID ID #REQUIRED><!ELEMENT SubElement2(#PCDATA)><!ATTLIST SubElement2 ForeignID IDREF #IMPLIED>My XML is :<?xml version="1.0"?><!DOCTYPE Root SYSTEM "External.DTD " [<!-- This works --><!ATTLIST SubElement2 Reference CDATA #IMPLIED><!-- This doesn't seem to work --><!-- I just want to add a new child-element to Element1 --><!ELEMENT Element1(SubElement1*, SubElement2*, SubElement3*)><!ELEMENT SubElement3(#PCDATA)>]><Root><Element1><SubElement1 ID="E1">stuff1</SubElement1><SubElement2 ForeignID="E1" Reference="A">stuff2</SubElement2><SubElement3>stuff3</SubElement3 ></Element1></Root>