To extend your custom shape with custom attributes you can put something like:
<ext_attributes>
<ext_attribute name="Integer" type="int" />
<ext_attribute name="String" type="string" />
<ext_attribute name="Float" type="real" />
</ext_attributes>
between the <shape></shape> tags. The effect will be
some custom properties in your object. They are editable by the properties dialog
and will be loaded and saved with your diagram. To programatically access them use
"custom:<name>"