If you want to add new line in the existing PO, then it is possible to add new line item. what is the status of the PO?
Normally (sometimes) after invoiced the PO, we can add new line item where PO gets changed the status from Invoiced state to Open order state. It is possible. so you can try this option also but as you said that you don’t want to have earlier order item then go with the process of below mentioned & as i said this step in my earlier post also.
If you want the existing PO and want to insert new item record then Add new line at the PO lines in same PO and while updating the packing slip, In the Posting packing slip form… Goto Lines (tab) select the “CLOSE” check box to the item which you did not received and update the packing slip. Close option will do, If an order line cannot be delivered or can be only partially delivered, select this check box to set the remaining quantity to zero.
But as per business scenario, I don’t prefer you to go ahead with the above processes because you said for every step you need to fallow Approval process. So, I suggest you to go with NEW PO and cancel the old PO. Its “SAFE”.