Display subcategories in filter dropdown list

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.5
-- PHP version -- : 5.5.38
-- Browser(s) name and version -- : 56.0.2924.51 beta (64-bit)

Hi,
I would like to display all subcategories inside my filter dropdown to search by countries and/or by continents.

I joined here some picture to show you my problem.

Dropdown list by continents is ok ( number "1" on the picture)
Dropdown list by country display only continents (number "2" on the picture , there is not subcaterogies displayed)

One picture is showing my categories and subcategories tree.

Can you help me please, is there an easy way to do that with some config tips or can I do that overriding some classes/components ?
Thanks

Thank you, I will override the class function getCategories.

There is sotmething wrong in my code when I extend hikashopFilterTypeClass, the extended functon is never call.

My /templates/mytemplate/html/com_hikashop/administrator/classes/filter.override.php

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

defined('_JEXEC') or die('Restricted access');
include_once $originalFile;

class hikashopFilterTypeClassOverride extends hikashopFilterTypeClass{
    function getCategories($filter, $datas=''){
        echo 'getCategories';die();[color=red] //nothing happened[/color]
    }
}


class hikashopSingledropdownClassOverride extends hikashopSingledropdownClass{
    function display($filter, $divName, &$parent, $datas='', $multiple='', $tab=''){
        echo 'display'; die();[color=red] //nothing happened[/color]
	
    }
}

class hikashopFilterClassOverride extends hikashopFilterClass{
[color=red]    //if I delete this, i got a error:
    //Fatal error: Call to a member function getFilters() on a non-object in /path_to_my_website/components/com_hikashop/views/product/view.html.php on line 129[/color]
}


?>