Traditionally, states go through the process of becoming a US Territory before earning the right to become US states. Alaska is a good example - my ex- husband was born in the Territory of Alaska before it became a state.

I believe this process would work for Canada. I would suggest, however, that Canada be brought in as a number of territories,, combining some provinces and splitting others as it makes sense. As an Alaskan, I would love being able to travel to "The Lower 48" states without leaving the US. And I believe Canadians make very fine neighbors! I would welcome this process, personally.