I'm Japanese and used to fly for one of the LCC in Japan and also taught classes for expat guys. To convert your EASA to JCAB with A320 type, you have to take one written exam which is air law, and that's it. To study for your written exam, I believe the airline will give you a ground school and probably a question bank to study. For sample questions, you can also goto JCAB website. I'll PM you for the link, because I cannot post any website address here (I got denied because I haven't posted here not enough... that's what it says
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The best way to study is to get the English version of AIM-J. You can also browse through the AIPs. Just search 'AIS Japan' and you can create an account for free. English version is available.
You have to go through ATPL conversion training which probably lasts about 3 months (with ground school, SIM, etc..). Since you are already EASA A320 rated, probably you will do a shorter syllabus (I don't guarantee on this).
Good luck, and stay healthy.