I forgot to but something in a review and would like to go back and add it. Is that possible?

I have done this successfully once.  I contacted staff and they allowed me to edit a review for clarity.  There is no system in place for you to do it automatically, you would have to contact staff directly and plead your case.

Go to Account Manager and click on "my reviews" and you will see an edit button next to the review.

If it is posted, then you will have to write to Administration and ask for a special dispensation.