The short answer to that question is only editors can create/add categories and sub-categories. New categories are created when there is an actual need for them, that is when editors find a sufficient quantity of listable sites indicating a need for further sub-category organization. If you have a lot of links that would fit a new sub-category, suggest them the parent category in question. If the editor processing them thinks they require a subcategory they will create it. Given that no one (including editors) knows who will process the sites and create any necessary categories, there really is no one person that the suggestion should be directed too.
Hope that helps.