Add Category in filters in category view

[panoss]

When I make a custom field searchable (PRODUCTS->Custom Fields-> click on Title of custom field->Searchable. And 'Is a list'=yes) a drop down box is added in the category view and I can filter products using it.
How about if I want to do the same with the categories? How can I do this?

(so, for example, when I choose Category-A and Category-B it will display me the products belonging in these two categories)

[Studio 42]

Category-A and Category-B is not possible to filter in Virtuemart.
YOu have to use a paid filter module or component.