Navigate to the Atomic Assessments menu.
Select New Assignment in the top right corner.
Select Start from an existing activity.
Choose an activity from the list of Activities that appears. This list includes all assignments you have created in Atomic Assessments, which in turn includes all Items in the assignments.
Edit the assignment name, assignment settings, and click Save.
Please note: This will not be a new copy of the activity. If you make changes to the activity and items using this method it will update the original activity as well. If you want a copy make sure to duplicate the assignment instead.