Thank you for the reply Trix. I am not sure whether keep lists would be sufficient to do what I am looking for - i.e., keeping any/all current tags in place, but add any additional ones found.
Perhaps I am missing an elegant way to do this... I tried adding *Genre to the keep list (so that it would keep any Genres already in place) but it looks like that does not work for keep lists. I guess I could try to individually list every one of the couple of hundred of genres in my collection on my keep list - but I was wondering if there isn't some easier way to do this.
Just to clarify what I am trying to do - I already have 'first-level' genres for essentially all of my music - so a song may have 'Jazz' as a tag, and that is correct. But if 'Bebop' - a sub-genre of Jazz - comes up high on Last.Fm, I'd like to have "Jazz; Bebop" as my genre even if plain "Jazz" was not one of the tags found by the script. In an ideal world I'd love to be able to specify the order of these tags so that I'd be able to specify that "Jazz" (genre) should always come before "bebop" (subgenre), which should always come before "hard bop" (sub-subgenre)... but I would be happy to just be able to have them all jumbled up on the tag anyway!