Set thumbnail and title for a post type archive (not post itself!)

Do you mean something like this?

 $args = array( 'labels' => $labels,
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'query_var' => true,
        'rewrite' => true,
        'capability_type' => 'post',
        'hierarchical' => false,
        'menu_position' => null,
        'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
    );

This code goes after the function that creates your CPT and before the register_post_type

i.e.

function my_custom_post () {
...
}

$args = array (...)
);

register_post_type ($cpt, $args);