The beginning of wnfull.rdfs looks like this:
<rdf:RDF
xmlns:owl="http://www.w3.org/2002/07/owl#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:xsd="http://www.w3.org/2001/XMLSchema#"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:wn20schema="http://www.w3.org/2006/03/wn/wn20/schema/"
xml:base="http://www.w3.org/2006/03/wn/wn20/schema/"
xml:lang="en-US">
<owl:Ontology rdf:about="">
So it looks like it's already an OWL ontology, although with an rdfs file extension. You should be able to load it as an OWL ontology regardless of the extension - or you can change the extension to .owl if you prefer.