You can specify a meta attribute in your mapping file.
Code:
<class name="Xyz" table="XYZ">
<meta attribute="implements">XyzInterface</meta>
...
Alternately, if you need to put this same interface reference in every mapping, you can drop it into your generation configuration file (generate.config.xml).
Code:
<codegen>
<meta attribute="implements">XyzInterface</meta>
<meta attribute="implement-equals">true</meta>
</codegen>
Check Chapter 15:
http://www.hibernate.org/hib_docs/reference/en/html/toolsetguide.html