Hi Luis!
In this case, you need to create a simple mod. Create the module according to the article here:
http://kb.x-cart.com/display/XDD/How+to+create+a+module
and then add an additional class to your module:
<Module-root>/View/Subcategories.php
This class will decorate the \XLite\View\Subcategories class and it will re-define the getAllowedTargets() {} method. It should return array('category'), not array('category', 'main').
Please, let me know if it makes sense to you. If it is over-complicated, I will try to explain it in more details.