There are a lot of companies and a lot of individuals who have done this. It is somewhat of a checkered past....
In the '80's this was a big business called Gray Market cars. Conversions, in my understanding are both harder and easier. The engines often times can be re-chipped with US specs. But there are specific parts that are only found on the Euro cars. Exhaust systems come to mind as well as bumpers.
There used to be (and maybe still exists) a "one time exclusion" program, where by an individual could bring in one car, but it did have to conform. Often the feds ask that a bond be purchased for the value of the vehicle until it is made US legal. This leaves the buyer under beware.
The European companies such as BMW and Mercedes have worked hard to make buying a NEW car easy in what is called their Tourist Delivery or European Delivery programs.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)