You seem to be stuck in the (IMHO weird) idea of a user story being always something which is created by a business analyst making an interview with a user.

Sorry, but this is just a thought model, a simplistic example of how a user story might be created, but definitely not the only or the only correct possibility (probably not even the most frequent one).

A user story is just a sentence or phrase which describes a requirement from the perspective of an end user . 

- Who writes this sentence down is completely irrelevant. It could be an analyst, users by themselves, a developer, or any other kind of stakeholder. The person just has to "wear a user's hat" (but don't take that literally neither).

- If the requirement is found by an interview, or during some brain storming session, if it is extracted from some existing documents, by analysing some UI of a competing product or completely invented by some ingenious person does not matter. Typically, it is a mixture of all these techniques.

- This applies also for the different parts of the sentence.

So do yourself a favor and don't overthink this.