I am not a lawyer, and this is not legal advice.
As i understand it, we're legally free to insert whatever we want into a game. The only questions are: whether you're creating a "derivative work" under copyright law; and whether a EULA that forbids "modifying files" could be enforced to strip you of your license to run the game. Of course, one of the major problems is that we often aren't just adding; we're modifying, creating derivative work textures, models, and so on. And even if you make something entirely original, you often have to edit a game file to make it load the new data, so does that count as a derivative work? Even if you don't have to modify anything, does the game as a whole count as a derivative work? I'm not clear on that. It would be interesting to see that go to court. That is, interesting, not necessarily good.
Now of course you can't redistribute models, textures, music, and other "assets" from any game, even from SR3 for use in SR4 or from Morrowind for use in Skyrim. That's a clear violation of copyright distribution rights, even though you're not profiting from it. Or ... it could be. The law doesn't lay down specific rules for "fair use", just guidelines. I don't think porting an entire environment from one game to another would qualify as "fair use", though, because of the scale. It would also be interesting to see this go to court, but i suspect the plaintiff would succeed.
I have seen proposals that skirt the distribution problem. You can create tools that export all the necessary data from a copy of the source game on the user's system and import them into the target game. There is nothing being distributed but entirely original code, and as long as it doesn't have to break any encryption (running afoul of the DMCA), it should be entirely legal as far as distribution is concerned. Then, i think you're back to the first paragraph: does the game count as an unauthorized modification of a copyrighted work? For one thing, it takes place only on your computer, and would be infeasible or impossible to prosecute. So there's that. The tool authors might be held liable for "encouraging" copyright infringement. Once again, that would be interesting to see in court, but i anticipate victory for the defense.
There is such a thing as damaging good will between publisher/developer and community. If Volition or Deep Silver wished to prohibit the importation of assets -- even setting aside the issue of distribution -- this forum would shut down any such mods. Oh, and a lawsuit can be expensive and stressful, so there's something to be said for trying to avoid those even if you think you'd win.
Why am i typing all this, when this has been discussed to death on the internet for longer than i can even comprehend? Maybe it's the beer. Wasn't sure if the horse was quite dead, so i had to beat it a little to make sure.