Not a good idea.
If the dog is getting decent food it shouldn't need any supplements.
Multi vitamins for humans are exactly that FOR HUMANS........
Most multi vits contain iron (for example) which can cause big problems for dogs, also remember that initially there may be no observable side effects, but over time the ingredients could build up in the dog.
Vitamins C, A, B6 and D can all cause toxicity in dogs.
So once again, NO, not a good idea.