There are two major importation hurdles that make it nearly impossible. I say nearly, because there are a couple of extremely restricted/expensive ways to make it happen.
The two hurdles are US compliance to safety codes and EPA.
For safety, you have to show that the vehicle meets every applicable Federal requirement, including for how the speedometer is marked, turn signal sizes, lumens of light, etc. Doing this is by documentation, not just riding up to your local NHTSA inspector. (Good luck finding one.)
Then you will need to do the same for your State safety inspection to get registered.
But the big hurdle is getting past the EPA, because basically only manufacturers have the resources to do and document the tests required. More than that, even the manufacturers find that a model passing non-US requirements may need to be re-tuned or changed to meet the US requirements. The Super Tenere is a good example of this, as the US ECU and muffler are not what people get in the rest of the world.
There are a few companies that you could find with Google who can do what you are looking for. It's largely done for show cars or for deep pocket collectors like Jay Leno. Essentially, they have to go through the same hoops as a manufacturer and in the end the vehicle gets modified frequently. Expect to pay more than the purchase price of a brand new Super Tenere for their work, so it is cost-prohibitive for a 660.
About the only other way is to be a foreign citizen and bring the vehicle to the US on vacation with the tags of the foreign country. If doing that and the vehicle gets sold in the US, whoever buys it will need to do the full NHTSA and EPA set of hurdles as soon as he tries to put any kind of US plates on. Most of these basically become expensive orphans because they can't be ridden on the street.
The only other way I know to do an end run is to find a model that has a US version with the same frame, then transfer all the non-US bits to the US frame. Some Fords owners have done this to use Euro parts for a unique look or to get turbo and other motor parts. Technically they are not legal on the US roads, but the VIN is what all the registries and such are tied to. Of course, you may be buying most of two bikes this way.
