Open your model, and then select Model > Extensions to open the List of Extensions.
Click the Add a Row tool and enter a name for the new extension file.
Click the Properties tool to open the new extension file in the Resource Editor, and create any appropriate extensions.
When you have finished, click OK to save your changes and return to the List of Extensions.
The new XEM is initially embedded in your model, and cannot be shared with any other model. For information about exporting your extensions and making them available for sharing, see Exporting an Embedded Extension File for Sharing.