By "xml schema", do you mean XSchema?  
If so, I have Java classes that convert XSchema->DTD and DTD->XSchema. However, 
they have not been updated for the current spec and don't support namespaces or 
the AttGroup, Model, Enumeration, or UnparsedEntity elements.  I hope to have 
new versions in a few weeks, but these will save you some time right now if you 
don't mind cleaning up by hand.
You can download the converters from:
http://www.informatik.tu-darmstadt.de/DVS1/staff/bourret/xschema/convert.html
-- Ron Bourret