How do you add a settings page to another menu?

You’d need to create the top-level menu page, and then the sub-menu after that.

Here’s an example:

function my_menu() {
  add_menu_page ( 
    'MVC Events',      // $page_title
    'MVC Events',      // $menu_title
    'manage_options',  // $capability
    'mvc-events',      // $menu_slug
    'mvc-options'      // $function
    );
  add_submenu_page (
    'mvc-events',         // $parent_slug
    'ATB Event Options',  // $page_title
    'ATB Event Options',  // $menu_title
    'manage_options',     // $capability
    'atb-event-options',  // $menu_slug
    'atb-options'         // $function
    );
  }
add_action( 'admin_menu', 'my_menu' );

Here’s the pertinent WP codex file, and one particularly on sub-menus.