Of course.
It's easiest to do it via the module system, and it certainly won't be save compatible (trying to load an existing save after adding the files will cause some item mismatches, usually wearing shoes on the head slot or something like that) if you add them to their respective sections to make them available for sale in shops, which I assume is part of your intention.
Theoretically you can do it via .txt edits, but because of constants declared there may be a number of errors with this method.