Anybody can add a podcast using the Add Channel link at the top of the page.
They use a single level hierarchy of topic categories. Since they have a lot of podcasts listed in the directory, it means many of the topic categories have lots and lots of entries, and therefore it will take awhile to wade through the listings to find one you're interested in. I think it would be better for them to use more precise categorization to keep the size of each smaller.
They also have a tag oriented categorization. The tags do represent a more precise categorization.