Yeah, most of the stuff is already there and it would just be another choice for the player. So I don't think it would be that hard to do. If they'd go so far though as to include "they" as an option (which I personally would love) I think it would definitely be much more work. I would be fine with choosing between the already existing genders though if singular "they" isn't possible. That alone would be amazing.
Also, it's sad to see that people are so extremely against something that won't even affect their game since the whole gender update is optional anyways. But everytime this is brought up people complain about it. I really don't see the point. Like I said before: It's an option, no one is forced to use it, but it would bring comfort to some players and representation is, like I also mentioned before, very important. For many people it's good to see people or even sims just like them. Why take that away?