Normally the description field is individual for every movie, it is difficult to image a bunch of movies having the same description. So the description field is to be edited individually.
If you want to have descriptions from filmaffinity, you can download information about the movie from filmaffinity first and then update it from imdb.
If you download information about a movie, and the downloaded description is different from what you have, the new description will be added to your text instead of replacing it. Furthermore, in the IMDB script settings, you can adjust what information should be downloaded.