The Thalmor are Elder Scrolls version of the Enclave, they can and never will be join-able, only the Aldmeri Dominion will be join-able, not the extreme party controlling it. They will just be destroyed by a far weaker looking force (empire) who get an extremely OP character such as the chosen one in Fallout 2.
Ah I see what you mean now. In Fo3 you can put the virus in the purifier, but this isn't joining with the Enclave, its helping the president they betrayed same with destroying the Brotherhood, it doesn't make them friendly. In New Vegas the remnants, don't really count as the Enclave as they support none of their goals and are just old soldiers who defected or left upon the West coast Enclave's defeat.
I like the thalmor as villans. I hate them in most other cases though. But I do wish I could join them. They Should allow That In tes vi. I would love to destroy all who get in our way as we become as we once were!