Wrong, forging a US Army document is a crime. Doing research on it now, will get back to you shortly. Furthermore, forging a US Army document that impugns other official records is also a crime. Forging a signature is a crime...the list goes on and on.
And that may be precisely one reason the "Killian" memos are ones he "wrote to himself" (and not on letterhead). While they concern military matters, they are no more official documents than would be his diary if it mentioned Bush's service.