How could I determine if a US patent is also protected in Canada?

Many questions - many answers: The US Patent document cannot tell you where else the case was filed. To search in Canada, you can visit the Canadian Intellectual Property Office website - http://www.ic.gc.ca/eic/site/cipointernet-internetopic.nsf/eng/Home You can also search at the European Patent Office website www.epo.org - and see what other countries the US Patent was filed in. Improvement inventions (your "add-on") might be patentable. Section 101 of the Patent Law says: 35 U.S.C. 101 Inventions patentable. Whoever invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title. However, the new invention must not be obvious to persons having ordinary skill in the field of the invention. If the add-on is a simple change that everyone knew could be done - it is likely an obvious change - and it will not be patentable. Finally, infringement of any claim in a patent is enough. Do not copy! Make your invention different.

Check the Canadian patent office registry you can find it with a search engine such as Google. Or contact the owner of the U.S. patent and ask if the invention has also been patented in any other county. Patent infringement involves impermissible use of one or more of the claims of the invention. You may be able to patent an incremental add-on if it does not impact the patented invention itself and if it is not an obvious idea.