One of the current research on the Semantic Web area is semantic annotation of information sources. On-line lexical ontologies can be exploited as a-priori common knowledge to provide easily understandable, machine-readable metadata. Nevertheless, the absence of terms related to specific domains causes a loss of semantics. In this paper we present WNEditor, a tool that aims at guiding the annotation designer during the creation of a domain lexicon ontology, extending the pre-existing WordNet ontology. New terms, meanings and relations between terms are virtually added and managed by preserving the WordNet’s internal organization.