Modify File Entity Reference child node and its attribute

Is there way to make File Entity Reference children to
be modifiable? 
My xml has external xml file
<!DOCTYPE reporter PUBLIC "-//CEB//DTD Reporter XML
V1.0//EN"
 "my.dtd" [
<!ENTITY external SYSTEM "external.xml">

]>
<root>
&external
</root>



external.xml has 
<external>
  <name>external</name>
</external>



Node externalNode = externalNode.getFirstChild();
externalNode.setAttribute("id","external"); //doesnt
work got the below error



org.w3c.dom.DOMException: NO_MODIFICATION_ALLOWED_ERR:
An attempt is made to modify an object where
modifications are not allowed.
	at
org.apache.xerces.dom.ElementImpl.setAttribute(Unknown
Source)
	
Thanks for the help
Dave