Would not that be vandalization?
Not sure how others think about this, but from my perspective? I don't think so...
I mean, I'm not telling you to redo content, add new pages or modify actors - well, on the pages in question there are almost no actors, uhm... I think what you wrote is true. The author would probably be adding them as well, if these chapters ever got a do-over - adding them won't do any harm.
Not to mention, you already improve the original stuff by coloring and choosing colors of your own volition if you have to. You also add details here and there. From my point of view this wouldn't be so different. It's just a better way to show who's talking without having to write names into every bubble...
As long as you keep the style and make them look as if they were naturally meant to be there, it should be fine.
But as I said before, this was just an idea that came to mind when I re-read those pages. It's completely up to you. Just thought you could try something that might help when working on your own stuff.