It is possible to export and import mags as long as the party in US holds proper permit/s.
They then mail the mags to you in Canada and you may be a subject to Duty/taxes.
I, personally, had mags exported from US and mailed to me in Canada, exporter had proper paperwork, I paid duty and sh$t on this side. Mags HAVE to be legal in Canada thou.
None of the laws were ever broken.