Completely agree with Peter; Kanban works outside of the “sprint” structure. Let me mention, as an example, how JIRA can be used by teams to combine both methods. Stories can be assigned to a specific sprint or simply set to the backlog (shown at the bottom of the screen). Some organization (teams) allow their more experience developers to select those stories they can work on during a specific sprint, letting them make their own decision depending on the amount of time left on the sprint and the priority of the work to be done. It is from the backlog where usually PMs or POs select stories and assign them to sprints, however.