We offer an import mechanism for quite a few recipe plugins but some websites have their own custom recipe solution. Those cases will require a custom recipe importer.
You can create your own importer starting from this example custom recipe importer code. When completed, place it in the /wp-recipe-maker/includes/admin/import/ folder and it should automatically be recognized and available on the WP Recipe Maker > Import Recipes page.
All our own importers are available in that same folder and can be used as an example. To learn more, we can also recommend reading this article by Bill Erickson.