Watch the video at community.getk2.org/opensocial/ningapps/show?appUrl=http%3A%2F%2Fos.ning.com%2Fningapps%2Fyoutube%2Fgadget.xml%3Fning-app-status%3Dnetwork&owner=3nkhqv24ick1c
on Content Uploader Pro at the bottom. It does both k2 items and joomla articles from xls type files
It and other import tools are made to go from articles-> items....
You can reverse this if you do a little work on the xls file by hand first.
With phpmyadmin create a xls file of articles
Whith phpmyadmin create an xls file of items
Look at the differences and arrange the xls of items with the same number and content in the columns that the xls of articles has.
use the import tool to import the file...
Test on a trial site as it may take a bit of trial and error.Pm if you need help