Content codes are warnings, yes, and you can change them. They become part of the summary field, so when you go to edit the story, you can select the edit details option, and then edit to summary field to include the new tag.
One caveat, though: that field has a character limit of 240 characters, so if the additional tag would go over that, it won't register. In that case, since most authors don't want to cut down on the summary itself, you can place any new content codes/tags at the top of the very first chapter. We don't count that as an author's note for the purpose of that word limit, either.
I hope this helps!