Take a look at the
cloneNode() method on the org.w3c.dom.Node class.
You can take the root node of the second DOM tree, clone it and attach it to a particular node in the first DOM tree.
Disclaimer - I haven't tried this myself, but it should work!
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.