Well I started the download, but I can't find/don't know where to dump the existing stuff into (I'm assuming I should be looking for a Witcher 2 folder somewhere in Steam > steamapps > common). Also, when I back out to Steam > steamapps > downloading, I see the three folders I had copied are already there with everything in them, which would have been at least 90% of the game data, but the download is still at like 1%, so I don't really know what to make of any of this, but it looks like something like this won't be so simple.
All of it is in downloading, so it won't appear in common. Go to steam>steamapps>common and then copy/paste your Witcher 2 files in there. Make sure it's in a folder named "The Witcher 2" without the quotations. Then verify through Steam.
Alright, after several attempts I think I can confirm that this doesn't work. I copied the files, but the only thing I could verify (again, I'm assuming you meant to go to "Properties" and click on "Verify Integrity of game cache") was the ~500MB I had downloaded. Steam wouldn't recognize the existing files no matter where they were or what folder they were in.