Extending a CPT by Created by another plugin

I am not sure exactly what you are trying to ‘extend’, or how you want to extend it, but you can alter posts types with add_post_type_support, remove_post_type_support, and a few other functions.

You can add metaboxes to the post type with add_meta_box setting the $post_type argument to the name used to register your post type. (Examples are on the Codex page) Unless the original plugin provided a hook you won’t be able to directly alter its metaboxes but you can use remove_meta_box to remove existing boxes and then add back a near clone of the metabox with your changes.

You will have to hook into save_post or admin_action_{$_REQUEST[action]} to save any additional fields you create.

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