I still don't understand what you're talking about. Doesn't the comparative fault have to be adjudged? Isn't that part of the verdict? A competent attorney should have the defendant identify in discovery any third party alleged to have been at fault - and those parties can be joined where warranted.
Are you really saying that a defendant can arbitrarily add a third party on a verdict form and that party - without representation or even a ruling on relative comparative fault as borne out in trial - is thereafter cast in judgment based in that defendant's designating the party as such?