User stories can be written by anyone. Although it is the product owner's job to ensure that an agile user story backlog exists, this does not imply that the product owner is the one who produces them.
During the early stages of product development, the team discusses needs and records them as user stories. As long as there is a product backlog, it will never be frozen. As a result, if someone thinks there's a missing requirement or anything that could benefit the client, they can add it to the queue as a user story. There is no rule or guideline indicating that the stories must be written solely by the product owner. Because there is a set format, anyone creating the story should know exactly what it means and how to write it.
