I don't
Some people will mix some pork or beef fat in, some add bacon. My wife won't eat burgers so I grind mine straight and then use it in chillie, sloppy joes etc. I have also heard it is better to freeze it straight and if you want to season it season it before you prepare it. The fat is the first part of the meat to break down so it will have a shorter freezer life if it has been mixed with something.