I'd import the data twice: Once into an invoice or PO table and once into a related line items table. You'll need to set up one method or another to prevent duplicate PO entries. One simple method is to set a unique value--validate always validation rule on your PO# field in your PO table. Then subsequent lines of your data with the same PO# should be automatically filtered out during import. Then I'd import the data for the individual line items into a line items table.
To see all the line items on for your PO on the PO layout, add a portal to the line items. This will give you a much better database structure to work with.