I'm pretty sure it isn't just a matter of a phyto; the USDA has a federal quarantine order relating to import of citrus from HLB infested countries, and Japan is on the list. You can't even import citrus seed from Japan into the US. A phytosanitary certificate is what you need in the normal case, but where there's a quarantine order, you'd need a specific import permit exempting you from that requirement. I would be shocked if USDA would issue an import permit for a citrus tree from Japan to a home-gamer, given the inability of an average person to prove that they could provide adequate containment of risk.