Flagged e-mail name

When I flag an e-mail and add it to my tasks, how can I change the task name (ie subjet) so I can clearly define the task and keep it associated with the e-mail?

If you meant changing name in Agenda in sidebar it is not possible because it is still an email and you cannot do it for emails. You can create task (with desired name) from email by menu Actions->Create Task from Message.