You can attach an extended model definition (.xem) to your model when you create it by clicking the Select Extensions button on the New Model dialog. You can attach an extended model definition to an existing model from the List of Extended Model Definitions.
You should never modify the original extended model definitions shipped with PowerDesigner. To create a copy of the file to modify, open the List of Extended Model Definitions, click the New tool, specify a name for the new file, and then select the .xem that you want to modify in the Copy from field.