libxml seems to treat the included external paramter entity as part
of the internal subset. I cannot tell, if that's correct, but it look's
logical at least.
So I suggest you create some 'newsml-nitf.dtd' which includes the
two dtd sets (NewsML and NITF) as external entities and reference this
dtd as the external subset of the dtd in your document.
greetings
Morus