Need help with Artist tag w/multiple files

So I'm trying to add an artist name to multiple files at once (that have various artist tags), WHILE KEEPING the artist name/tag that is already there. For example, I'm tagging some music from the Grand Theft Auto games. I want to be able to add "Grand Theft Auto" to the artist field, while keeping in tact the multiple artist names that are already there. I know how to do this when changing one file at a time, the problem comes when I want to add another artist to multiple files at once that already have varied artists, without deleting the names/tags of those multiple artists in their respective fields. When I select multiple files at once that have multiple artists (from within Mp3tag), the "artist" field will display "keep". (I just noticed that this post will not allow the arrow symbols for some reason. When I use them, the word in between them does not show. So just assume I'm using the arrow symbols that show in the Mp3tag box). So I tried to add the artist name while keeping the "keep" syntax in tact, but that didn't work, it just came out "Grand Theft Auto "keep" as the new artist tag on every file. This is not what I want. And I would rather not have to go through each song line by line to make this change, as I plan on making similar changes elsewhere in my fairly large library. Can someone please help me figure this out? Thanks

See e.g. the FAQs:

Alternatvely to the suggested action, you can also use Convert>Tag-Tag

The big advantage using Convert Tag -> Tag is the immediate preview
(green rectangle in the screenshot):


Please keep in mind that "Grand Theft Auto" is not an ARTIST name. This could confuse players to think that your current artist content combined with "Grand Theft Auto" could be a new artist.

And also don't forget that you can not undo this step, if you don't use a unique separator character between the current content of ARTIST and your manually added text.

Yea I plan on separating it with \
(man I swear this website is weird. It won't show certain symbols correctly. I just typed and posted two of these \ symbols but it only will show one)

The whole point is I want Grand Theft Auto to appear as an artist within my media player (WMP12 mainly & sometimes Foobar)

So if there is a song by Hall and Oates, on the GTA soundtrack, I want to tag the song with 2 "artists".

But you got me exactly to what I was looking for. Thank you very much. I don't know why you say that change is unable to be undone. I just tried and you can certainly use the "undo format" button to reverse the change.

THis is only true for the current sesssion.
As soon as you quit that and re-load the files then the data in ARTIST looks like 1 string.
A unique separator between the individual artists is a good idea.

Thanks for you guys' help. Saved me a LOT of time lol

Just enclose such characters in back ticks or press the </> button in your answer or press Ctrl+E for the selected characters or text.

Two backslashes look like this \\