If I were to take the characteristic, "non-binary," and define it as, "not fitting to the characteristics of what is expected to be normal of the binary set of male or female biology or mentality," which I'm sure is a sufficient generalization for the sake of getting to the fucking point as most people seem loathe to do these days, then I can apply non-binary to a wide swathe of characters.
Yes, we have aliens whose sexes and their biology or mentality don't really correspond to ours, our "normal", so that wouldn't make them non-binary in their society even though for us they'd basically be traps, shemales, trannies, etc. Yes I use those terms, get over it. We even have one or two who naturally have more than two sexes if I remember correctly, so those don't count either. These examples are not what I mean of non-binary already being in. Just covering my bases here, don't want anyone to get offended or get the urge to be a couch professor on fictional gender dynamics and lecture me, now do we?
No, what I mean when I say "They're already in" I mean there are characters who used to be entirely human and are in one way or another intersex or have become that way eventually, or are part of a species which does as a matter of codex lore fact share the normalcies of our sexes and have also become or already were intersexed in one way or another as a matter of choice or lack of it. If they were once human or any other species that shares the same spectrum ends, and have modded or find themselves at a point of being a mix of the two spectrum ends to hermaphroditism or intersexuality of any flavor, then yes, they are in fact non-binary and several characters fall under this scenario.
Now, why pray tell do you not see them or find them hard to track down then? Because most have already changed themselves to the point where they are happy with themselves and don't make much of a fuss about it anymore, at most you'll get one conversation section for the majority of them. Started with one body or gender and now find themselves happy in another. One off the top of my head that is still in that process is Embry, who transitions easier/faster due to player influence if they help them.
So yes, it is quite sufficient to say, "they're already in."