If you figure out the right US postage cost to send the postcard to Canada, you can do it. Owl post is just a fancy "postmark" that they put on before dropping the letter into the US mail. I have brought my own stationery and some older US stamps with owls on them and it was adorable.
I can't recall if they sell stamps there (I think they do) so you can just put some extras on there to get to the right postage cost.