word/phrase for items added to an active contract
When you change a contract like that, it's called modifying or amending the contract. Each set of terms added together is called an amendment. (It's also sometimes called an appendix, addendum, or rider, but those can also be portions added before the original signing.) If the amendment specifies tasks to be done, I don't know a specific word for such tasks, but you can say "The Contract contains thirteen tasks, ten in the original contract and three in the amendment(s) thereto."