There is one way that it is possible for a new category to be created as a suggestion from "the outside".
Find at least 10 sites that could go in such a new category. Submit all but three of them to the category where you think the subcategory should be located, and then apply for that category using the last three sites as your examples, stating that you are applying for a new category, what it is to be called, and alert the reviewer to the other sites already submitted too.
It would be rare for this method to result in exactly what you are expecting, but if the sites are relevant and listable, then very little harm (to the directory) could come from that course of events.