1. I don't think it's NHTSA that cares, it's the DOT. Way back when, the way to deal with this was to have US-spec'ed parts (bumpers, instrument panel, lights, etc) installed on the vehicle here. Once installed, the car became a US car as far as the DOT was concerned.
2. The EPA also an issue. Once again, in the old days you would install any additional US-style pollution equipment necessary (air injection, cat, whatever). You then had to get the car to an EPA-approved lab (not your local emissions test station) and have it tested. This could cost a few thousand bucks and there is no guarantee that it would pass. If it did, you were set by the EPA.
3. You would have to pay a customs fee. I forget, 15% of the value of the vehicle? Something like that.
If you remember the movie 'Rainman', Tom's character was importing Ferraris for resale, basically doing the same thing. I read an article in the paper a few months ago about folks locally trying to bring in Porches, MBs and other high end cars that aren't sold here. Their problems were (as I remember) with the EPA taking months to deal with their certifications during which they could not license or use their cars.
Bottom line: It can be done, but it's expensive, and generally not worth it for cars like this.