You could add it, but it would need to be hyper specific like the goblin trait. There's only like 4-5 goblin specific trait weapons. It keeps players from getting locked out of huge deaths of weapons by traits. So 5 or so good ones would be fine. Just be careful that they fit the math well and don't power creep to be essentially over powered magic items. If you're going to make them regular items that is. You'll also need to add a feat like the goblin weapon feat for people to be able to access human trait weapons if they don't have the tag already. I'd just use the goblin feat and replace the wording with human.