The BSD's don't have the namespaces/unshare syscall functionality like Linux does.
Use microvms instead of containers perhaps?
FreeBSD has jails, which AFAIK are fully a match in terms of functionality, but you would need to finish implementing containerd support (maybe via runj?) and then get k8s to run on top of that. I seriously doubt there's any actual technical blockers, it's just that nobody's finished implementing all the pieces and putting them together.
No, FreeBSD has jails but k8s uses different types of runtimes for OCI containers (containerd, CRI-O, Docker Engine, etc).