You can't assign a Joomla! template in an item or category not in Joomla articles nor in K2 articles. That it how Joomla! works I'm afraid. You can do that only through a menu item.
K2 uses templates but K2 templates so not quite sure if this will help you with what you want to achieve.
You can view these videos
getk2.org/documentation/k2class2012 to understand a bit more about K2.