Can I trade in a Hong Kong-purchased iPhone 15 in Canada?

Yes you can, but I must warn you. If you choose to trade in your iPhone in a different region, you have the choice of a Gift Card that can only be used in that region, or a new device which will be a model intended for sale in that region. One of the issues with that is warranty which isn’t guaranteed internationally, you may have to return to Canada for service on the iPhone. You may also face compatibility issues such as eSIM is not supported in Hong Kong. You will also face differences in features and restrictions.

Apple will have access to the necessary details when you enter the serial number for the trade-in offer.

I won't swear to Canada, but probably... if it was the US, you could, so long as the device is functional and not activation locked or physically damaged.