I think nobody can't help like that way. Because it's illegal. Only U.S. Military and the family member can use PO on base(actually people who has PO box/address on base).So if you are not Military/family member, you should use Japanese post office. Even you have friend who in Military/family member, please Don't ask to them. It's illegal, but some people don't know thats illegal, then help friend and get trouble. So if you ask to your friend about it, you are going to give Trouble.
By the way, this question(looking for that help) is kinda no good for on this like public/SNS thoght. Please just use Japanese Post Office.