You can create a Menu item called Categories for example, or 10 menu items each one for your top categories.
Then you can start adding submenu items, for example in the top menu level called Contact add the Email sub-categorie. So when users visit your website can navigate through the menu to all categories you got.
An other way to do it faster is to use this plugin: http://www.nonumber.nl/extensions/addtomenu but I think it only works for items and not for categories (but you can ask the developer).
IMPORTANT: You can have top-categories configured to show only the items of this one or also with sub-categories, and can create a K2 menu item which can display in one link lot of categories you can choose in the parameters using CTRL+CLICK (even from other top categories).