Just like RMS assigns the next available # when adding a new customer, RMS should have a "next available #" for auto assigning the next Item Looup Code when adding inventory. At it's most basic, all you would do is give the starting number , say 10000.
You could always overwrite the number suggested with a manufacturer's 12 digit upc.
The implementation guide talks about "automation for numbering items", let's see it!