Can you really alter the editor used when typing data in 'extra fields' within the K2 content component for Joomla?

When pasting large swathes of content from MS Word, clicking 'Save', then re-opening the content, the information is finished.

No, it's not easy to alter the editor. And why would you like to alter the editor? What is you are attempting to input?

It's possible, simply not through configuration parameters. I edited the 'extraFields' view (admin -> components -> com_k2 -> views -> extraField -> view.html.php) and added a Javascript include to load another editor.