Yes you are correct on the PO it does not allow you.  The work around is that when you setup your item in inventory, if you go all the way down in item setup, you will see "Purchasing Information"  What I do is for example is: (123) [Removed] .  So this way when you do purchase order the SKU will pop up as (123) and then the description.  Its the only way to show your SKU to put it in the Purchasing Description.