Adding tape or pine tar to a bat are not considered altering the bat. They are put on a bat for grip purposes and are both legal as long as they don't extend more than 18 inches from the handle end of the bat.
Corking is a means of drilling out or hollowing the barrel end of a bat and filling the hollowed area with cork (or any other substance). This allows the batter to generate more bat speed since the bat is now somewhat lighter at the barrel end and also gives the barrel a more "spring-like" action than it would have with solid wood. Corking the bat is flatly illegal.
The rules do not specifically state that weight can't be added to the knob end of a bat. (the rules don't say a lot of things) However, this alteration would be used for only one reason: to change the balance of the bat enabling the batter to generate more bat speed without having to "choke up" on the bat and I would consider it to be an illegally altered bat and would not allow it to be used.