Item Properties can be added/updated through Administration > Setup > Inventory > Item Properties.
Once the window is opened you’ll notice a list of Item Properties (note scroll bar below). Although additional item properties cannot be added, there are 64 pre-titled Item Properties that are waiting to be renamed:
Simply overwrite the name of the item property with the title you prefer. Once they are updated you can then attach these new properties to the items you wish to associate them with.