Without a sponsor from a Japanese airline, I do not think that you can convert on your own, at least I have never heard of it being done.
When I worked at NCA, current & qualified type rated 747 pilots had to go through the complete 6 month JCAB training program at the Haneda training center. There was no short cut, no differences training.
My only suggestion is to contact the folks at HAC to see if they can give you words of wisdom, they deal with NCA & JAL.
http://[email protected]/index.htm