Just open a given RIF/XML ontology as if it would be a native ObjectLogic ontology:

OntologyManager oblOntologyManager = ...;

oblOntologyManager.openOntology(ontologyURI, parameters);

The ontology URI in a RIF ontology is identified by the annotation ID of the document formula. If it has none, the physical URI is used as the ontology URI. As usual; if necessary you will have to register an ontology to a physical URI mapping in the ontology resolver of the ontology manager.

Alternatively use

oblOntologyManager.importOntology(new File(physicalURI), ...);

to import a RIF/XML ontology without the need to register an ontology to a physical URI mapping.