Add states to countries that do not include states in WooCommerce core

custom-country-states-woocommerce.php

<?php

/**
 * Add custom states to country in WooCommerce (Ex: Nigeria)
 */
add_filter( 'woocommerce_states', 'NG_woocommerce_states' );
function NG_woocommerce_states( $states ) {

	$states['NG'] = array(
		'SS' => __( 'Some State', 'woocommerce' ),
	);

	return $states;
}