You won't be able to add extension data to the categories if the categories are being added via the XPath in a product mappings node (like the snippet you have posted here). However, you can later reference the category using the XPathLookup like you've specified in the product mappings
Code:
<Entity EntityType="Category" XPathLookup="/cat/cat-1" Action="Update">
<ExtensionData>Your ExtensionData here...</ExtensionData>
</Entity>
or you can add when you create the entities outside of the products
Code:
<Entity EntityType="Category" Action="Add">
<Name>Category Name</Name>
...
...
<ExtensionData>Your ExtensionData here...</ExtensionData>
...
...
</Entity>