Installed Forca Theme, wonder how I can alter Post editing screen

I've just installed Forca Theme on my fresh WP installation.

They add a few custom taxonomies like:

1 Events Categories - Just categories for Events

2 Events - Same as regular Post, with custom behaviour and template, have assigned Events Categories to it

3 Events Page - Lists ALL Events, no possibility to display from particular Events Category because the Event Page is not associated with ANY Category, it's the Events (Items) themselves that are assigned to categories.

So what I want to do is to add the same Category assignment box from Events to Events Page screen which looks like this:

And is assigned like this:

register_taxonomy("events-cat", 
                        array("Events Categories"), 
                        array(  "hierarchical" = true, 
                                "label" = "Events Categories", 
                                "singular_label" = "Events Categories", 
                                "rewrite" = true,
                                "query_var" = true
                            ));

register_post_type( 'events-item',
    array( 'labels' = $labels,
         'public' = true,  
         'show_ui' = true,  
         'capability_type' = 'post',  
         'hierarchical' = false,  
         'taxonomies' = array('events-cat'),
         'supports' = array('title', 'editor', 'thumbnail', 'comments', 'custom-fields', 'page-attributes') ) );

In /includes/register.php so I tried to add a new taxonomy registration to POST screen according to documentation I've found:

register_taxonomy("post-events-cat",
    array('post'),
    array(  "hierarchical" = true,
        "label" = "Post Events Categories",
        "singular_label" = "Post Events Categories",
        "rewrite" = true,
        "query_var" = true
));

But well, it didn't work.

So I wonder how I can tweak particular screen in Wordpress? Not even really related to the Theme itself.

Thanks