I have a custom post type that is created by an external plugin. This plugin uses the capabilities associated with posts. I want to add my own custom capabilities to this post type. I have seen plenty of examples of how to do this when registering a new post type but I can't add the capabilities at this point as otherwise the custom capabilities won't be there when I next update the plugin.
What would be the best way to add the capabilities after the custom post type has been created?