You'll get me in, right?
Co-sign. 15 hours a week working =/= legitimate excuse for poor grades. You've had an upward grade trend, however, which is excellent. The adcomms will note this. If your cum GPA is just barely above a 3.0, it might be worth mentioning something. If you want to include an addendum, I suggest it say something along the lines of:
I appreciate the committee's consideration regarding the upward trend of my GPA over the last four years. Adjusting to college life proved to be more difficult than I had anticipated, and this was reflected in my first year grades. I am proud of my accomplishments and have been consistently recognized on the Dean's List for the past X semesters. Having beat this early set-back, I feel confident that I am prepared to manage my time effectively to be successful in a law school setting.
Just keep it short and sweet. Addendums don't need to be more than 1-2 paragraphs.