[capink]

You can use the template language to populate a custom column with the user categories.

program:
    user_categories()

You can update the column with calibre's search and replace (which supports using a template as an input field), or by using the Action Chains plugin.

Also, the template language should allow you to control what user categories should be included and what should not. For example, if you want only user categories starting with "Nationality." to be included:

program:
	res = '';
	for cat in user_categories():
		if '^nationality\.' in cat then
			res = list_union(res, cat, ',')
		fi
	rof;
        res