You'd have to package FEX :D
For the kind of person who wants to run NixOS on Apple Silicon or do Linux gaming on Apple Silicon in the first place, that's probably interesting and not too hard
but if you're allergic to that, you might be able to figure something else out with Box64, which is already packaged in Nixpkgs
x86_64 gaming on NixOS is of course well supported and has been for a long time. There's a 'native' package that I've always used and the Steam Flatpak is also available and works as well as it does anywhere
we are talking about asahi linux. i think it is pretty clear that nixos isn’t supported like a first class citizen because you have to do a fair bit of work to make all of the more recent userspace fixes work on NixOS. i run NixOS Asahi so I know.
it was easier when Arch was a first class citizen but the advice nowadays you get upon encountering a problem on Arch is to switch to Fedora