The general problem here is that Tasks and Stories fall into different domains. Tasks are created and modified by Team Members, while the Product Owner is responsible for creating Stories.

So we have two basic possibilities (while avoiding violating Scrum principles):

1. The PO can create Stories from Tasks, but leaves the original Taks untouched. A Team Member would need to delete or set the Task completed.
2. A Team Member converts the Task to a potential Story. Since he may not write into the Product Backlog, in the logic of Kunagi, the Task would be converted to an Issue (that the PO can then proceed to convert to a Story).

I think there's a third option that might be simpler to implement:

The PO can split a story into another part, leaving the tasks in the old story. Then the team can move the relevant tasks into the new story. I think this could all be done from the whiteboard, and the whiteboard can be used to create tasks for future sprints as well (with a dropdown at the top to select the current sprint or any not-closed sprint). The only thing missing would be to be able to transfer tasks in between sprints. Also tasks left over when the sprint is done should not be deleted but rather should be transferable to a new story in the same way...

What do you think, it's a good idea?

Thanks for the quick reply by the way!