You don't need to modify anything. Just create menu links pointing to multiple categories. That's all. Similar options exist for the K2 Content module as well.

When you retrieve items from multiple categories (via a menu item created in the backend) you will notice that there are additional "display" options available to you. This happens because in such cases, we need to be able to override basic display settings for retrieved categories, as individually these categories might have different display parameters already shared.