Add Featured Image to Custom Post Type

I have a plugin that creates a custom post type, named popup. But it does not add the Featured Image Meta Box on the edit page. I can go in the plugin and add it like this.

'supports' => apply_filters( 'popmake_popup_supports', array( 'title', 'editor', 'revisions', 'author', 'thumbnail' ) ),

This works, until the plugin is updated.
In My theme I have added this:

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add_theme_support('post-thumbnails', array('post', 'page', 'popup'));

But that does not ad the Meta-Box to the Custom Post Type.

Question: Is it possible to add this feature outside of my plugin, maybe in my functions.php file, so I can update the plugin?

EDIT:
Ended up using the free plugin Advanced Custom Fields, which let med add more then one featured image. http://www.advancedcustomfields.com/

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1 comment

  1. Now you are using the right code in functions.php file.It will works.
    Please follow my instruction and use below line of code in functions.php file .
    Here is the code.

    function custom_theme_setup() {
        add_theme_support( 'post-thumbnails', array('post', 'page', 'popup') );
    }
    
    add_action( 'after_setup_theme', 'custom_theme_setup');
    

    Now if still not works then please specify the priority of actions.
    See here.

    Like this :

    add_action( 'after_setup_theme', 'custom_theme_setup',2 );