Add custom taxonomy to default category taxonomy?

Question

I'm trying to add a custom taxonomy to default category taxonomy but I could not figure out how to.

I tried this but failed :/

register_taxonomy(
    'oyuncu',
    'category',
    array(
        'hierarchical' => false,
        'label' => __('Oyuncular', 'series'),
        'query_var' => 'oyuncu',
        'rewrite' => array('slug' => 'oyuncular'),
        'show_ui'           => true,
        'supports' => array('thumbnail'),
    )
);