I admittedly don't really know how SSH is built but it looks to me like the patch that "makes" it HPN-SSH is already present upstream[1], it's just not applied by default?
Nixpkgs seems to allow you to build the pkg with the patch [2].
Upstream is either OpenBSD itself or https://github.com/openssh/openssh-portable , not the FreeBSD port. I'm... not sure why nix is pulling the patch from FreeBSD, that's odd.