Custom or Additional Fields in Purchase Order Detail

For each purchase we make, we would like the ability to add both comments/notes and a GL code for reporting purposes.  We would like to either have these fields listed so we can add these on a per item basis in the PO, or better yet, add Custom Fields in the PO detail section.  I realize there are custom fields in the PO header, but need something that applies to each purchased item, as often items that are purchased on the same PO have different GL codes within the same PO.