Vehicles are added to that list based on application made by an importer or simply the initiative of the officials that regulate the list.
To be imported, a vehicle has to be either 1) substantially similar to a US spec vehicle of the same year, or 2) capable of being modified to meet the requirement. The Defender 110 has a VSP number, which means that it is importable because there was a 1993 NAS Defender 110. Presumably you would have to add the NAS equipment like the roll cage.
Note that the NHTSA rules only apply to the car itself, not the engine. The engine is regulated by the EPA. Thus, as long as a 1993 Defender 110 diesel could pass the importation emissions test, it would be importable. Naturally, the best approach would be importing a v8 set up with NAS emissions system components.
I have intimate experience with this process because I imported a 1995 BMW 540 Touring (station wagon) from Germany in 2002. The 5 series wagon was sold in the states, and the 540 was also offered, however, there was no US spec 1995 540 station wagon. The car was eligible for importation as being "substantially similar" under VSP 194 and the egine passed emissions. So in that respect I was able to "mix and match" to get a car that was never sold int he states.
Importing stuff that way (where very few modifications are required) cost on the average, 10k per vehicle (assuming it passes the emissions test).