This extension support to import and export XMI (XML Metadata Interchange) for StarUML. It is based on XMI 2.1 and UML 2.0 metamodel. Diagram interchange is not subject of XMI.
XMI (XML Metadata Interchange) is for metadata interchange among various software modeling tools. However, XMI functionalities of many tools are not complete or not compliant with XMI standard. Therefore, you need to aware that XMI is not perfect way to interchange data.
Select the menu: File > Import > XMI Import (v2.1)
Select the menu: File > Export > XMI Export (v2.1)