My understanding based on a discussion of how it works for Canadians here (I went to the Nenkin Jimusho with a Canadian colleague) is that with proof of having paid into the system in Canada and then finishing out your career in japan for example you would add the say, 15 years in Japan (not enough to meet the minimum 25 years paying into the system)
From next month, the minimum in Japan will be reduced from 25 years to 10. Some 640,000 people who weren't eligible under the old system will now become eligible for a small pension, even though the promised income tax rise to accommodate it has yet to materialise, putting further strain on this already creaking system.