@Mxyzzy I can't help with downloads from other sites. However, if you have an Origin-controlled install, all the packs you've installed also need to be registered in Origin, or they usually don't work. Sometimes people can install an unregistered pack via disc, and Origin accepts it, but that's likely because the user also inputs the code that came with the disc.
If you did buy a sealed disc copy of KPST, you could simply register the code in Origin and download it from your game library, the same way you would if you'd bought it directly through Origin. But a used copy would probably come with a used code, so you wouldn't be able to register it.
