Hello,
High level stakeholders are identified as part of the Project Charter. During Initiating, both the Charter and the identification of stakeholders is taking place. If you look at the PMBOK 6, (p. 81) it specifically says that a key stakeholder list is created as part of the Charter. Because the Charter is created using the Business Documents and provides a high-level understanding of the project, the Charter can then drive the development of the Stakeholder Register. Having said that, new stakeholders are ID'd throughout the project.
Christine