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't
change 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>