I'm making Boston Cream puffs. Can I add vanilla extract to the vanilla cook and serve pudding to make it taste more vanilla?
Sure you can. However, there is a limit to how much vanilla flavor tastes good, and without more information there's no telling you when that limit would be reached; in fact, the existing vanilla pudding may already be there.
You can test this by adding a smidge of vanilla to a spoon of pudding and then tasting it. However, be careful; after multiple tastings it could be easy to get used to the vanilla and end up putting in way too much for non-habituated tongues.