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>"