using entities

Hello list-users,

my problem is that I have a DTD including lots of entities for greek  
characters, special characters and so on. I need them in my schema.  
I´ve read that every entity must be written under schema as a separate  
element. Is it possible, although DTDs are not xml syntax, to  
reference from a schema to the DTD entities? I tried to convert some  
entities to schema using oxygenxml dtd => schema but the entities get  
lost, as expected. Is there a suitable solution under schema?

Thanks for helping me,
Andreas