I think hand coding the DTD is not really an option for large schemas.

I think hand coding the DTD
is not really an option for large schemas.
"Correct"

There is no direct way for the conversion but you can generate an XML 

instance document from each XML Schema using the action Generate Sample

XML Files from menu Tools. For generating a DTD from an XML instance 

document run the actions Learn Structure (Ctrl + Shift + L) and Save 

Structure (Ctrl + Shift + S) available on the menu Document -> XML Document.