Not really, Apple would just have to buy a GPU chip from NVidia or AMD, put in on a board with a Thunderbolt controller (Which they should be more than capable of doing, see the Thunderbolt Display), put it in an enclosure and help create the appropriate drivers.
NVidia and AMD wouldn't have to do the Thunderbolt integration at all since Thunderbolt is basically just external PCI Express.
Somehow I don't see this happening. Apple has no interest in getting into dealing with graphic cards and making external thunderbolt ones. Also, I'm sure NVIDIA and AMD would be more involved then just handing Apple a desktop or mobile gfx and letting them build it into an external model...I am sure there would be more involved. I think we are quite aways away from reasonably priced external gfx cards that can match the performance and power of current cards.