Business domain ontologies offer great opportunities for facilitating communication between people in business, for improving the enterprise system engineering processes and for creating interoperability between enterprise systems. However despite these opportunities, their use in practice is still limited. This can be partly attributed to the lack of formal representation of these ontologies. This paper proposes a structured approach which uses conceptual models as intermediary representation for formalizing business domain ontologies. The proposed methodology is used for the process level specification of the Resource Event Agent Ontology.