Rather than adding a cemetery as a new subcategory under the top-level "Cemeteries" category, first consider what you want to use the category for. Is it for info about the cemetery itself? Or is to keep track of "Where My Ancestors are Buried"?
There are intriguing, unusual facts about cemeteries -- their history, the famous people who are buried there, cultural differences in burial practices, horror stories, headstone inscriptions, etc. For some people, that may be the reason they are interested in cemeteries.
For other people, their interest is that they "collect" cemeteries (like Jim Tipton, the guy who started FindAGrave.com, for instance).
But maybe, like me, you just want to know (and keep track of) "Where are the graves of my ancestors?".
Drilling down from the top-level "Cemeteries" category, there are abundant links to cemetery lore. And yes, one can categoize ancestors into 'their' cemetery -- but they would share that category with everyone else who is interred there, not only your own ancestors.
What I created, and what I'm inviting the WT community to use, is a place (category) for grouping one's OWN ancestors in a given cemetery. Here's how.
From the top-level list of categories, click on "Family". From there, click on "Graves". At present there are only two categories in "Graves": one for *my* ancestors (named "Graves of STRAEHLE-3's Family"), and one for testing purposes ("Graves of SUPERMAN-1's Family"). I invite the WT community to create their own category here. For instance, a subcategory created by FOGELZUP-17 might be named "Graves of FOGELZUP-17's Family". Jimmy Ringding might create "Graves of RINGDING-42's Family". Chris Whitten might create "Graves of WHITTEN-1's Family".
Next step is to create a subcategory for each cemetery where there's an ancestor. About 20 of my relatives are buried at The Evergreens Cemetery, another dozen at Green-Wood Cemetery, a few at Calverton National Cemetery, another bunch at Oakwood Cemetery, etc. So within my "Graves of STRAEHLE-3's Family" subcategory, there are further subcategories: "Graves in The Evergreens Cemetery", "Graves in Green-Wood Cemetery", "Graves in Calverton National Cemetery", etc.
These are the, uh, terminal subcategories. Once they are created, 'populate' them with profiles.
For those WTers who are already adept at categories, that may be a sufficient explanation. For those WTers who'd like to categorize your relatives into cemeteries but not quite sure how to accomplish it, give a holler and/or a comment below. Help is but a question-mark away!