I think it might harm you in the end. Usually the addendum is used to explain a discrepency or provide clarity. Normally when you use the addendum to explain an LSAT, you use your GPA as an evidentary counter to explain your scholastic aptitude and potential in law school. Remember, law schools receive hundreds of these addendum statements and they usually come off as complaints and excuses. If you have what is essentially an excuse for your LSAT AND GPA, the ad com will begin to question whether or not your worth it as a student if you have an explanation for every little problem. Saying that you didnt care about grades would look poorly on you as a potential student. True, you can say that in law school you will care or work harder, but so does EVERY student in that situation. They can read from your resume that you were productive in other venues other than academics. If I were you, I would be as descriptive as possible on the resume. It's not a business resume to where you have to be totally succinct and have neat little bullet points; a law school is going to appreciate a short resume, but are going to prefer a resume where activities are explained to an additional addendum that they are going to have to read.