you won't be able to get canadian insurance until after the import process is complete. the process goes - start import at border - inspection - finalize import - insurance - registration - done.
you can drive your us car in canada under us insurance for as long as your us insurance company will allow. usually when they find out you are importing it, they will give you 6 mo and drop you. fyi the only way they find out is if you tell them. they won't find out through the import process until it's completed. this really doesn't matter because you only have a limited of time to finalize the import and register your car after getting inspected. it's somewhere between 60 and 90 days if i remember correctly.
