Well, the short answer is yes. You can't use a already copied files on your mod.
The long answer is that one could make a mod installer that uses the old files, so that it wouldn't be in discord with the copyrights, but you would have to be a genius at coding to do this.
Fortunately there is some good news, some one has already made this happen, the bad news is that it was at a different game, a little better news is that it was an Interplay's game, namely the Baldur's Gate(BGT). The twist at this is that, the original design was at breach of the copyright(in a way, quite like your would be, thought there was a code that would check the originality of the product), but it was made by the original makers of the game and thus, it was kinda legal, and thus it was soon discarded by most wed sites(as there hasn't been much in game support), but modders have recently refined a mod to uses the original content so now it isn't in breach of the copyright and it is much smaller.
Yes, I know that the above message might be more than a little confusing, but I am aliterate, so sue me, I can't help it.
