When stricter noise limits are enforced to legacy wind turbines already deployed, actions need to be taken. In this paper, we present a solution of retrofitting wind turbine blades with additional outer layer skins that change their aeroacoustic footprint. An optimization design framework produces add-ons shapes that, when attached to blades, reduce their noise without compromising aerodynamic performance. The Blade Element Momentum theory is used to predict the aerodynamic performance and generated noise is predicted using semi-empirical models. Two competing metrics are analyzed, Annual Energy Production and Overall Sound Pressure Level, using a multi-objective genetic algorithm. The add-on shapes are parameterized using NURBS totaling 54 design variables. The AOC 15/50 wind turbine is used as a test case and optimal solutions selected from the Pareto front are discussed. The after-market add-on approach produces solutions that range from an increase of 8.7% in energy production to a decrease of 3.5 dB(A) in noise levels, with an estimated blade weight increase of less than 4%. While the add-on approaches typically fall short in terms of performance when compared to a new blade design, this retrofiting is expected to be a competitive alternative when compared to the cost of replacing the whole blade.