You can add a data type to eCore models by manually adding an eClassifiers
entry to the *.ecore file. (I was unable to find a mechanism to do this through the Eclipse UI.)
For example:
<eClassifiers xsi:type="ecore:EDataType"
name="MyClass" instanceClassName="my.package.MyClass"/>
This will make the class available as an option for the EType in the Properties editor for EAttributes. It does not appear to make it available for EReferences.