Grimaldi, there are millions of Scots scattered all around the world who still consider themselves as Scottish, however long they have been away. There is no conceivable way in which a method could be found to allow them to cast a vote on Scotland's future.
Once an immigrant arrives there and gets his/her name on an electoral register, they are entitled to vote...and rightly so...however hard it might seem to someone who has, effectively, forfeited that right by leaving.