I'm pretty confident that if Canada did choose to join ballistic missile defence, in its negotiations it would ask for a seat at the table. At the moment, NORAD has confirmed that it has no obligation to defend Canada against a ballistic missiles, because NORTHCOM would, in fact. That's at the commander's discretion. I'm pretty confident that Canada would have a seat at the table if it were to choose.
On May 5th, 2016. See this statement in context.