Hi Louisthehun,
While there is no official Edit in Excel function for the Item Journal, there is an easy way to do this without using an add-on. It simply requires the use of the Microsoft Dynamics Office Add-In in Excel, which is the same tool that the Edit in Excel functionality uses.
You will need to create a Web Service for the Item Journal page (from the Web Services page, click on New, Object Type = Page, Object ID = 40, and give it a unique service name like ItemJournalWS, and click Publish, and click in another row), and then utilize that information in the Microsoft Dynamics Office Add-In in Excel.
I tried to record a short video but ran into an issue with it today when attempting to publish records (keep getting a 'data object not provided' error). I know that it worked just a week or two ago because that's how we imported all the Item quantities and balances for our most recent client.