The first week of groundschool is spent doing Air Law and at the end of this you do the Air Law exam at a local JCAB facility. After that it's radio law and radio theory, followed by a locally administered exam in each subject. All this is followed by the usual suspects: dangerous goods, safety equipment etc.
Once you've finished that stage it's onto the aircraft systems and the written 767 type exam. If you've already got the 757/767 on your licence then there is no need to do this bit.
Next comes fixed base sims, followed by the full motion sims. If you already have a type rating then you go straight to doing the ATPL test in the sim (examined by JCAB). If you're not rated then you first do your type rating exam in the sim (examined by JCAB) and then you do your ATPL test, as above.
Once you've done your medical and English test they will apply for your Japanese ATPL and, after a brief respite, it's line training time.
So, the answer to your question is 'yes', you do have to convert your non-Japanese licence to a Japanese ATPL but 'no', you do not have to do ALL the exams again. The whole course is designed around achieving this and you'll come out of the end of the sausage machine as a 767 rated pilot, flying the line with a Japanese ATPL.