You can modify a model by editing its XML file using a standard text editor such as Notepad or in an XML editor, but you must be very careful, because even a minor syntax error could render the file unusable.
If you create an object in an XML file by copying an existing object of the same type, make sure that you remove the duplicated OID. It is better to remove a duplicated OID than try to create a new one because this new ID may not be unique in the model. PowerDesigner will automatically assign an OID to the new object when you open the model.