From my understanding, you have to:

- either get the EObject from the semantic model (ie the set of EObjects) referenced by your .aird file

- or if the EObject has been initialized manually and is not already referenced in your .aird file, attach the resource containing your EObject (ie the file where your EObject is serialized) to the session

- or if the EObject is not intended to be serialized at all, use Sirius's transient attachment mechanism

Get the EObject from a resource referenced by an .aird file

Collection<Resource> resources = session.getSemanticResources();
  /* Iterate through resources */
  for (Resource resource : resources) {
    /* Iterate through EObjects */
    for (EObject eObject : resource.getContents()) {
       ...
    }
  }

Attach the resource containing an EObject to a session

session.getTransactionalEditingDomain().getCommandStack().execute(
  new RecordingCommand(session.getTransactionalEditingDomain()) {
    @Override
    protected void doExecute() {
      /* The eObject must be contained in an XMI resource */
      /* See § "Data Management" here => http://eclipsesource.com/blogs/tutorials/emf-tutorial/ */
      session.addSemanticResource(eObject.eResource().getURI(), monitor);
    }
  }

Attach a transient EObject to a session

eObject.eAdapters().add(new SessionTransientAttachment(session));

Hope it helps,

François