Most apps provide some method of being able to export your recipes. CookBook supports most industry standard file types so if they do export files (other than their own proprietary files) then we might be able to import them.
The more descriptive recipe formats are JSON-LD, Microformat's hRecipe and Schema.org's Recipe. If you put either of these in a text file, CookBook will be able to read the content of the data.
Within the app you'd just need to hit 'Add recipe' then 'From a file' and you'll be able to select the file from your file manager on your phone.
You could also try using the online clipper tool and see if that works with you old apps website. Just use your CookBook account details to login to; https://thecookbookapp.com/signin Once logged in, you can access the clipper ad import the recipes from websites.
Unfortunately, all recipes must be imported one-by-one on both the app and the website. We working to release a bulk import option sometime this year.