Hi,
It's not easy as the data is not loaded in the emails.
You'll have to edit the emails via the menu System>Emails and in the "preload" area, you'll have to add first a MySQL query to load the category_name of the categories in the hikashop_category based on their category_id that you would load based on the category_id of the entries in the table hikashop_product_category where the product_id value would correspond to the id of the product that you can get in the $product->product_id variable there. Then, you'll be able to loop through the results of your MySQL query and display the category name after the product name.
If you're a developer it should be quite easy to do.