How to add support for category excerpts?

How do I add support for excerpts for post categories in the admin? I have tried the following, but to no avail:

add_post_type_support('category', 'excerpt');
add_post_type_support('post_category', 'excerpt');
add_post_type_support('taxonomy', 'excerpt');

It's got to be something like this, but I just don't know.