I suppose that you intend to use Modelio to create a Class Diagram with some classes and then export this diagram to EMF UML which can be imported in Overture. Overture will then create VDM classes matching the class defined in the class diagram.
To convert a UML class diagram model to a VDM++ model, you first need to export the UML
model from Modelio to the Eclipse XMI format, called UML using the EMF UML3.0.0 format.
At the moment, Modelio is the only UML tool supported. In particular, the exported UML models
have been tested using Modelio 3.3.1.
Exporting: Right-click a VDM++ or VDM-RT project to access a submenu for UML Transformation.
From here it is possible to Convert to UML. The resulting .uml file will be saved
to the generated folder of your project.
Importing: To perform a UML import you must have the .uml file in the relevant project folder.
You can either copy it manually or use the Eclipse Import - File System feature. Afterwards,
it is possible to right-click the .uml file and choose the submenu for UML Transformation
and then select Convert to VDM
See the Overture userguide for further information