This is going to be very dubious advice, as I’m just some American living in Canada who has kind of looked into this myself. I honestly don’t really know what I’m talking about but here’s how I understand it.
First, you can just leave your retirement accounts in the US. Start a new retirement account when you get to Canada. Then just let both accounts grow separately.
Second, it is possible to move a US retirement account to a Canadian RRSP, but you’ll probably have to pay a 15% - 30% tax to the US when you do that.