For instance, I have:

     fileSystems."/zfs/media" =
        { device = "tank/media";
          fsType = "zfs";
        };

in my hardware-configuration.nix. tank/media is defined as using a legacy mount-point or whatever the ZFS terminology is. Done.

ETA: I mean, I had to do all the gruntwork to get the pool built, yeah. But once it was defined, getting it mounted and all the kernel bits and bobs set was trivial like that.