Q: can a canadian iphone be registered in the usa?

Yes. The fact its sold in Canada has no bearing on whether it can be added to an Apple ID.  It only has bearing on which cellular carriers it may be able to use, but since you don't need it to have service, its a moot point.

Make sure the device is correctly unlocked so you can add it to your account:

what-to-do-when-buying-second-hand-iphone-ipad-or-ipod-touch

I may receive some form of compensation, financial or otherwise, from my recommendation or link.

You can direct the seller to:

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

So they can release the iPhone form their account if they have not done so.

Note, a valid Sim card, from the carrier the iPhone is locked to if its locked to any carrier, or just any carrier if its unlocked, will be required for activation.

You can remove the sim card once the iPhone has been activated. The sim card does not need to have active service on it.