You can add your desired currency in the plugin using the filter provided by the plugin itself.

For this, you need to activate the child theme and after the activation of child theme add below-given code in your child theme’s functions.php file.

/**
 * Wp_travel_custom_add_currency_val
 */
function wp_travel_custom_add_currency_val( $currency ) {

    $currency['ZMW'] = 'Zambian kwacha';

    return $currency;

}
add_filter( 'wp_travel_currencies', 'wp_travel_custom_add_currency_val' );

/**
 * Wp_travel_custom_add_currency_symbol
 */
function wp_travel_custom_add_currency_symbol( $currency_symbol ) {

    $currency_symbol['ZMW'] = 'ZMW';

    return $currency_symbol;
}

add_filter( 'wp_travel_currency_symbols', 'wp_travel_custom_add_currency_symbol' );