This issue is not limited to smart dogs. It's also a problem for clever cats. Unless a pet has the clever or smart trait at the start, you cannot change it and it will simply show a missing third trait in their offspring if they were meant to inherit the trait.
I've also noticed that neither clever or smart show as personality traits when you try to add via debug. Where "smart" should be in the menu, it just doesn't exist. You can add it in full edit CAS, but as others have said, it won't stick. Unless a dog or cat are spawned or created with the traits from the start, it is impossible to give them to them, and the offspring cannot inherit them. It's frustrating, and it's been a problem for as long as I've been playing. Tonight I decided to see if it was a known bug (with the hopes maybe someone had made a mod for it).
