Many people may find K2 component of Joomla now does not have JCE editor, TinyMCE and other 3rd party editors in its extra fields. K2 now is using NicEdit, which does not have many functions like DocMan link.
According to the announcement of K2, this was for forbid a potential unstablility of K2. The guys at K2 say they have not included the JCE editor as it causes some kind of performance issues if multiple instances are used - "because there were conflicts calling multiple times the default editor set in Joomla, be it TinyMCE, JCE or whatever. So we added NicEdit to avoid these issues.". However, we provide a hack solution here to add JCE into K2. Please use it carefully and backup your code of K2 first before you do any change.
The version we tested is K2 v2.4.1. In the file: /administrator/components/com_k2/models/extrafield.php at line 208, replace:
$output='id.'" id="K2ExtraField_'.$extraField->id.'" rows="10" cols="40" class="k2ExtraFieldEditor">'.$active.'';
$editor =& JFactory::getEditor();
$output = $editor->display("K2ExtraField_".$extraField->id."", $active, '550', '400', '60', '20', false);
If you meet any problem or have any question, please feel free to contact us.