add_action('admin_menu', 'edit_trustees_page');
function edit_trustees_page() {
    global $submenu;
    $trustees_page_id = 1; //change this value
    $url = get_admin_url() . 'post.php?post=' . $trustees_page_id . '&action=edit';
    $submenu['index.php'][] = array( 'Edit Trustees', 'manage_options', $url );
}

index.php => Dashboard
edit.php => Posts
upload.php => Media
link-manager.php => Links
edit.php?post_type=page => Pages
edit-comments.php => Comments
themes.php => Appearance
plugins.php => Plugins
users.php => Users
tools.php => Tools
options-general.php => Settings