I wouldn't add polygons in SU if the model is going to ultimately end up in 3ds. Add your extra polys in 3ds with its excellent poly modelling tools just so that you know what you are getting as your end result and with much easier and cleaner results. But you can do it in SU using the Artisan plugin and its subdivide or knife tool depending on how much control you want over where the new polys go and how they are organized. Artisans knife tool is one of the least recognized of the best SU plugin tools. Just be sure to use it in orthographic views and probably with your perspective turned off for the most predictable results.
