It wouldn't work because only people who would have negative contributions would join.
For universal health care to work the cost of the care must be spread out among everyone. According to a WHO paper on universal health care:
A key element of financing for universal health coverage is sharing
resources to spread the financial risks of ill-health across the
Canada nor any other country can force Americans to join their universal health care so people would need to join voluntarily.
The Americans who earn enough to fund the universal health care system won't join. They can already afford health insurance in America and their premiums would only go up by switching because they are needed to fund those who can't afford it.
This would result in only Americans who can't afford insurance joining and Canadians paying more to cover them. It would be a great deal for the Americans but a terrible deal for Canada.