We are currently using "original time estimate" instead of "story points" and want to use time estimates for story points but are only allowed to input the estimates once the sprint starts. In addition, when we start the sprint, a notification pops up saying that x number of issues have not been estimed (the number of sub tasks). Then when we add them after it is tedious and are treated as scope changes.
How can we allow time estimates for sub tasks in the backlog before the sprint begins?