You need to include these files as part of a mod of your own, editing the art defines files (specifically the unitmembers file) to use the new model. The art defines files are hidden inside one of the .fpk files, so you need to use Nexus to unpack them first. I believe the ones you want will end up in the Common directory.
If you're replacing an existing unit's artwork, then you'd just need to edit one of the existing lines in that defines file. (But remember, although these are XML files they're not GameData files, so you can't use <Update> logic. You have to copy the entire file, and turn on VFS.)
If you're adding a new unit, then you'll need to create a new entry (obviously, copied from the original version) in the unitmembers file, and you'll also need a new entry in several other art defines files as well, such as the main artdefines_units file.
If you have no idea what I was talking about on any of the above, then you might want to reconsider the whole idea. This is fairly advanced modding we're talking about.