Is it possible to add categories to a Custom Post Type in WordPress?
Put the following in your theme's
functions.php file (changing
my-custom-post-type to be whatever your Custom Post Type is, of course):
I found this link to be helpful with the full function call and hooks to target, it even mentions the distinction between if the custom post type was created by a plugin: http://pippinsplugins.com/add-already-registered-taxonomy/