The additem.cnk available on Customersource / Partnersource works fine with GP2010 R2 and GP10. In the download page the compatible versions are listed as GP 8 and GP 9, but it works with later versions.
This.cnk forces the ‘Add Item’ functionality to be ‘on’ when a user opens the Purchase Order Entry window. This is important where users want to know if the Item they have added to a line on the PO is an Inventoried or Non Inventoried item in GP. With the Add Item flag switched on, the user is presented with a message asking if they want to set up a new item, when they enter a Non Inventoried line item.