Yes, while normally schools are suppose to report the full amount of tuition paid on the 1098-T in box 1 or the amount billed in box 2, if your school has not, you can add the additional amounts paid. During the 1098-T entry, you should see a blue link underneath box 2 that details what if this is not the correct amount - click that link and a new box will open for you to input the correct amount of tuition paid. Also, this is just for tuition and fees - there will be a separate entry screen after the above for books and materials.
Note: You will want to make sure that you have some sort of records or proof that you actually paid these amounts when entering your educational expenses this way - something such as canceled checks, a record of your account with the school showing dates paid, or credit card statements, etc.