The US missile defense efforts aren't really aimed at countering the North Korean "threat". They are concerned the most about the Russian nuclear potential. Canada joining that program would likely make it less secure overall, because this will upset the Russians, who naturally view the American strategic missile defense systems as a part of the nuclear first-strike capability. That's why Moscow has consistently protested the deployment of such systems in Eastern Europe (allegedly, against Iranian "threat"). Canada should consider the fact that, unlike North Korea and Iran, Russian ICBMs are both reliable and able to penetrate sophisticated air defense systems. In case anti-missile batteries are based in Canada, say, similar to NORAD monitoring stations in the Arctic, they will be the first to be targeted by the Russians.