You can reverse engineer XML schema files to add objects to an existing XSM.
Open the XML model you want to reverse into and then select Language > Reverse Engineer schema File to open the Reverse Engineering dialog box.
Select the file to reverse, and specify any appropriate options.
[optional] Click the Target Models tab and specify any existing PowerDesigner models which are referenced in the file being reverse engineered. These references will become shortcuts in the reversed model.
Click OK to begin reverse engineering.
A message in the Output window confirms that the file has been reversed and the Merge Models window opens:
Review the objects that you will be importing, and the changes that they will make to the model (see Core Features Guide > The PowerDesigner Interface > Comparing and Merging Models).
Click OK to merge the selected changes into your model.
[optional] Select Symbol > Auto-Layout to organize the new symbols in your diagram.