public Foo  createFooObjectFromString(EDataType eDataType, String initialValue)
	{
		return new Foo(initialValue);
	}

    private static class XMLResourceSetFactory implements IResourceSetFactory {

        @Override
        public ResourceSet createResourceSet() {
            ResourceSet resourceSet = new ResourceSetImpl();

            resourceSet.getPackageRegistry().put("OldPackageUri", ConfigPackage.eINSTANCE);

            resourceSet.getResourceFactoryRegistry().getExtensionToFactoryMap()
                    .put("xml", new ConfigResourceFactoryImpl());
            return resourceSet;
        }
    }

		ResourceSet resourceSet = resourceSetFactory.createResourceSet();

		// Do not register ePackages if not set.
		if (ePackages != null)
			register(ePackages, resourceSet.getPackageRegistry());
...



	private static void register(List<EPackage> ePackages, Registry registry,
			Map<String, EPackage> backup) {
		for (EPackage p : ePackages) {
			String nsURI = p.getNsURI();
			EPackage old = (EPackage) registry.put(nsURI, p); --> override my package impl
			backup.put(nsURI, old);
			register(p.getESubpackages(), registry, backup);
		}
	}

   <extension
         point="org.eclipse.emf.edapt.factories">
      <factory
            class="org.example.MyEcoreFactory"
            nsURI="http://www.eclipse.org/emf/2002/Ecore">
      </factory>
   </extension>

@Override
	public Object createFromString(EDataType eDataType, String initialValue)
	{
		switch (eDataType.getClassifierID())
		{
			case ModelPackage.FOO_OBJECT:
				return createFooObjectFromString(eDataType, initialValue);
			case ModelPackage.OTHER_OBJECT:
				return createOtherObjectFromString(eDataType, initialValue);
			default:
				throw new IllegalArgumentException("The datatype '" + eDataType.getName() + "' is not a valid classifier"); //$NON-NLS-1$ //$NON-NLS-2$
		}
	}

	@Override
	public EObject create(EClass eClass)
	{
		switch (eClass.getClassifierID())
		{
			case ModelPackage.MY_CLASS: return createMyClass();
			case ModelPackage.MY_OTHER_CLASS: return createMyOtherClass();
			default:
				throw new IllegalArgumentException("The class '" + eClass.getName() + "' is not a valid classifier"); //$NON-NLS-1$ //$NON-NLS-2$
		}
	}