Since you don't want to create NA and JP versions of each server, why not give each server a 'pairing.'

For instance Durandal(JP) and Ragnarok(NA) could be 'paired' servers. If NA players from Durandal want to be on a NA server, then people from Durandal all can transfer -only- to Ragnarok. If JP players from Ragnarok want to transfer to a JP server, they can transfer -only- to Durandal. In this way, everyone stays with their friends and they don't have to try to get everyone together.