Hi,
The category explorer can be displayed from anywhere in HikaShop with the function hikashop_setExplorer once you initialize the HikaShop main helper.
For example:
<?php
if(!defined('DS')) define('DS',DIRECTORY_SEPARATOR);
if(!include_once(rtrim(JPATH_ADMINISTRATOR,DS).DS.'components'.DS.'com_hikashop'.DS.'helpers'.DS.'helper.php')) return true;
echo hikashop_setExplorer('product&task=selectrelated&select_type='.$this->type.'&control='.$this->control,$this->pageInfo->filter->filter_id,true,'product'); ?>
That is used for the display of the explorer in the popup for selecting related products with the old interface of the product edition page (file 'selectrelated' of the view "product").
So it's not impossible to have it displayed from a 3rd party module, but it's not that simple as the first parameter is supposed to be a HikaShop controller. It will require some tinkering.