<foo xmlns:DC="http://purl.org/metadata/dublin_core">
<DC:Creator>Eric Hellman</DC:Creator>
</foo>
then the meaning of DC:Creator is not affected by any additions made to
Dublin Core. Your DTD remains valid, simply not as extensive as the
(expanded) Dublin Core.
This is certainly not a solution to all the issues that might arise when
combining elements from multiple namespaces, but it appears to work fine for
the example you cite.
Hope this is helpful,
Andrew Layman