Yes, a hand warmer could equally be used on the feet to keep you warm. This is because both the hand as well as the feet design operates with the same mechanism to keep the body warm and they are designed with more or else the same materials. The only difference between the two items is that the hand type is not designed in such a manner that it would fit well inside the shoes.
These types of products are bulkier and they do not have any adhesive element to help fix them in place, unlike the feet warmers. Therefore, they would be very uncomfortable on the feet something that may ruin the exciting moments during your activities of the day including hiking and ski trips.
Hence, instead of relying on these types of the gadget as a presumed substitute make sure that you use the right gear all the time. If a particular gear is to be used on the feet, make sure you use it on the part of the body you are required to use the gadget on if you wish to gain the full benefits of your product.