Editing Items

Sorry if this is a dumb question, but I looked around the board and didn't really see an answer. I'm wondering how to edit items in the main single player game, specifically adding attribute changes. I'm playing an elf and have maxed out his strength, but since elfs are -1, I can only have a 19. I want to use a greatsword and a shield, but it requires a 20 str, so want to add +1 str to an item. Thanks for your help.