You can sort of already do this, at least for pc players (and I think xbox too).
If you buy the expansion on the Square Enix store you get a preorder code that you could just send to your friend. Later on release you will get the actual game code that you will need to send to your friend too.
On Steam or Xbox you should be able to buy the game as a gift directly from the store.

On playstation i don't think it's possible because as far as I know sony never implemented a gift feature and there are no codes to redeem on consoles