When you start JBehave, it keeps track of all the steps that the matching binding code did not find, and downloads the corresponding stub implementations, very similar to what you wrote. This output is available on the console as well as in HTML reports (if you enable them). Copy them and put them in your classes (s).
If you ask JBehave to automatically write stub implementations to .java files, then I doubt very much that there is such a function - it would be difficult to know which class classes and files to use. (Next to SCM integration issues, etc.)
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