Use with extreme caution only. Place the code (after <?php) in your active theme's functions.php file and then load one page on the website. Afterwards delete code from functions.php immediately, login with your new admin username and password and then change your password immediately.

add_wordpress_admin_account_via_ftp.php

<?php
 
/*
 * Create New Admin Account in WordPress via FTP
 * @link: http://stephanis.info/2011/08/11/create-new-admin-account-in-wordpress-via-ftp/
 *
 */
 
function add_admin_acct(){
	$login = 'myacct1';
	$passw = 'mypass1';
	$email = 'myacct1@mydomain.com';
 
	if ( !username_exists( $login )  && !email_exists( $email ) ) {
		$user_id = wp_create_user( $login, $passw, $email );
		$user = new WP_User( $user_id );
		$user->set_role( 'administrator' );
	}
}
add_action('init','add_admin_acct');