Can you put varnish over polyurethane?

Yes you can. You should always check for compatibility of products, but generally an epoxy coating can be top-coated.

Link to Sherwinn Williams Macro-poxy with top-coat recommendations:

http://www.paintdocs.com/webmsds/webPDF.jsp?SITEID=STORECAT&doctype=PDS&lang=E&prodno=B58W610