References to separate ontologies for subclassing

[Ivan Filippenko]

I have two ontologies, in separate projects, where certain classes in one ontology would logically extend (subclass) classes in the other ontology. I'd like to implement this, but can't figure out how to make it work in Sparx EA. The Enterprise Architect User Guide says, regarding the OWL Relationships "Imports" toolbox icon (in the ODM Web Ontology Language tools), that "Imports" is "An extended UML PackageImports, that enables an OWL ontology to reference another OWL Ontology." But I have seen no documentation on how actually to use this icon, nor does that description convince me that it would necessarily fit my purpose. I'd greatly appreciate it if someone could describe how to make a class in one ontology extend a class in another, particularly if the two ontologies reside in their own separate .eap (project) files -- having been developed by different parties, for example. Thank you!