I don't know anything about Americans transferring their credits over to Canadian schools, but I can I tell you that if you have a Bachelor's Degree in ANYTHING, you DON"T have to take 2-3 years of prerequisite courses to get into nursing. You can spend one year completing your prereqs and apply to accelerated nursing programs in Ontario (these programs are about 2 years).
If you don't have a Bachelor's but have 2 years worth of university credits, you can still apply to accelerated nursing programs.
I have a Bachelor's in Animation (totally NOT related to Nursing!), and I simply completed all of my prerequisites (Microbiology, Psychology, Statistics, Philosophy, Anatomy & Physio) in one year. It's do-able, you just have to study a lot!
While completing my prereqs, I applied to York University's accelerated nursing program....and I got in!!! I really didn't think I would have a chance but somehow I got in.
Accelerated programs also look at your cumulative GPA and your GPA in the last 5 full year courses. I didn't think my cumulative GPA would be high enough, but I did do really well in the prereq courses.
So my main point is, you don't need 2-3 years to complete your prereqs, and you don't have to apply directly from highschool.
I hope this info somewhat helps!