Adding Meta box to thrid-party plugin

Register the metabox for just one post type with add_meta_boxes_$post_type:

add_action( 'add_meta_boxes_event', function()
{
    add_meta_box(
        'wpse_118766_box',
        'Custom Box',
        'wpse_118766_render_box' // function to render the box
    );
});

function wpse_118766_render_box( $post )
{
    echo 'hello';
}