A couple of years ago now (3? 4? ) there was a short report in the local (Alliston) paper of a possum found in Barrie, suffering from frostbitten toes. From that I'd inferred that they didn't winter well up here but if they're in Wiarton (northern Bruce Peninsula) they obviously do get by!
As to the how, I heard this story years ago "from a guy". (Sorry, can't recall the source and never tried to verify it. ) Apparently some of the fruit trucks bound for Canada - peaches were mentioned - are loaded at night in Georgia and so on and the possums stow away on board. Accidentally, one assumes. As I said, I never tried to verify this but it would explain how they get across the Niagara frontier, a substantial barrier and one which is currently keeping a new and nasty strain of rabies confined to the US and out of Ontario. For what it's worth.
Saw my first live possum this July past here in Alliston but have been seing roadside corpses for half a dozen years at least.
Peter