The rules in Canada are the same as in the USA for being able to work
in Canada.
You must have a right to work here.
1) Canadian Citizen
2) Landed Immigrant
3) Work Visa
4) Marry some canadian....
Now if you wish to convert to Canadian ratings it is as follows...
1) Go to a Canadian transport office to get a private pilot license
based on your FAA rating.
2) Take your commercial written exam. Fly with an instructor to
get signed off for the flight test. A 172 is all that you have to fly.
3) Take the Instrument exam. Fly with instructor and take the
flight test.....This time in a Twin.
Now you have a commercial multi, group one instrument.
Now you can apply and work at a canadian company. As long as
you have a right to work in Canada.
There you have it

. I was actually looking into this (I got this from someone who did it) because my significant other is canadian. I wouldn't do it to go fly for a regional in CAN, but flying for Air Canada would be sweet.
I'm also looking into Qantas, the process and requirements look similar down under.