Hi, 

You could do what I just did. I customized the PO template and used the "Other" column and named it Item #. You could name it whatever you choose. Check off both Screen and Print. You will probably have to adjust the layout as it will add an additional column. I placed this "Other" column in the "1" position. Now when I enter a PO, I can place a line number before each entry. Hope that makes sense. I attached a screenshot.