function eg_extend_subscription_expiration_options( $subscription_lengths ) {
	$subscription_lengths['year'][10] = wcs_get_subscription_period_strings( 10, 'year' );
	$subscription_lengths['year'][20] = wcs_get_subscription_period_strings( 20, 'year' );
	$subscription_lengths['month'][48] = wcs_get_subscription_period_strings( 48, 'month' );
	return $subscription_lengths;
}

add_filter( 'woocommerce_subscription_lengths', 'eg_extend_subscription_expiration_options' );