Hoi,
No. There is no such thing as "category namespace" in Wikidata. There are items for categories. They are related to other categories through the interwiki links but they are not linked in any other way to other queries. The data from categories is harvested but not consistently so. When you define what to expect in a category you will find that when you query Wikidata, Wikidata has different items because it has more relevant items and, there are items in the query that have not been categorised in the category.
No, there is nothing special about categories. If anything, through queries we can maintain the categories as well. That would be a boon for our readers.