If I buy an iPhone in Canada can I activate my USA Verizon carrier?

Only if the carrier in Canada will unlock the SIM card. They don't have to do this if they don't want to.

A Phone purchased in Canada will not be serviced in the U.S. You will have to ship the phone back to Canada for warranty service.

If you plan to use the phone in the U.S. most of the time, best to buy a phone in the U.S.