Here’s the current definition, from the 2014 CFLPA-CFL Collective bargaining Agreement (didn’t change this year):
6. The following Players shall be classified as National Players:
(a) Canadian citizens at the time of signing the Player’s first C.F.L. Standard Player Contract or Practice Roster Agreement;
(b) A Player classified as a National (formerly non-import) prior to May 31st, 2014; or
© A Player who was physically resident in Canada for an aggregate period of five years prior to attaining the age of 18 years.
The previous import rules were a bizarre definition based on years living in Canada, and where they received their early football training. I think there are currently two players who are grandfathered in according to rule (b), one of whom is Josh Bartel, who really pushed that definition. He is Australian, and never played North American football until he was in his twenties. But because he had never played football in the U.S. (obviously), he was deemed a non-import. I can’t remember who the other player is.
As mentioned earlier, the rule you referred to that allowed American players to become naturalized non-imports (playing in the league for long enough) was eliminated years ago.