J2XML 3.3 is the ultimate solution to import/export content in Joomla! 3Allows you to import and export articles and users to an xml file.
JA K2 to com content migration plugin is a FREE Joomla plugin to migrate K2 content to Joomla com_content in one go. It can convert all K2 elements: K2 categories, K2 Items, K2 Extra fields group, K2 extra fields, K2 tags. For K2 extra field, you can rename it if necessary.