Like many other schools in the USA, we also provide combined degree programs for MD degree students, but we do not do it the other way (i.e., PhDs have to "apply" for admission into the MD school, and be "accepted" into the MD school, to earn an MD degree).
Depends whether you're a Canadian living here or a Canadian only on a study visa. They accept PhDs from Canadian schools - the only other requirement is to be a permanent resident of the U.S. If you are a Canadian and part of the latter, you meet the program requirements.
Some MD-PhD websites have FAQ question about this and allow applications only during first year of Phd (maybe seen 1 or 2 programs).... there's also an SDN "Physician scientist" page FYI if you want to post there as well.