Allow admin accounts to reassign tasks

Currently once a task is assigned, only users that have been assigned the task can delegate another user to complete it on their behalf. Alternatively someone with admin access can complete the task on behalf of an assigned user. It would be good if an admin-level account could add another assignment manually.

Some back ground for this is that we have a workflow where tasks are assigned to groups of users responsible for that step of the process, but Nintex won't allow us to assign the task to a group, so we lookup a group in SharePoint, iterate through that group, assigning the task to all members of that group at the time the task is assigned. We had an instance where a new user was added to the group after tasks were assigned, so needed to add them to the task assignment (as they were now responsible for those tasks), we have to use delegation to do so, rather than an admin doing a re-assignment.