JDOM and external entities

[Tom]

I need to modify a DocBook file with several other XML files included
as an ENTITY reference. Like

<!DOCTYPE book SYSTEM "PathToMyDTD" [
<!ENTITY file1 SYSTEM "file1.xml">
<!ENTITY file2 SYSTEM "file2.xml">
<!ENTITY file3 SYSTEM "file3.xml">
]>
<book>
&file1;
&file2;
&file3;
</book>

JDOM's normal behaviour is to build this document with all entities
expanded. But how will I save the individual files after modification?
That's why I turned of entity expansion with setExpandEntities(false)
and build each file as a separate document. Now I am faced with the
fact, that some of those external XML files contain itself some of
those entity references declared in my root document. But without the
declaration I get a "Entity ... referenced, but not declared" error.
How should I handle this?

__
Tom