Yes you can. However, it's not an awesome idea. The e39 5ers are a bear to work on as far as the audio package goes. The head unit in a good audio system is the most important component, and in your car, you can't replace it (unless you want to spend mucho dinero). So, the best upgrades would be to add an amp and small sub to give you some low end, and replace your stock speakers with aftermarket speakers that are efficient enough to be powered by the stock amp. Adding an aftermarket amp to a oem head unit requires a line level converter that dirties up the signal a good bit. That's fine for subs, but your mids and highs would certainly suffer. Good luck, and lemme know of any questions I can answer along the way.