A few options:
1) If you're using the Java GUI, pull up the monitor for the schedule, and either drag-n-drop a new task into the schedule, or do a right-click > Modify schedule > Add task. The task will be added to the active schedule only -- not the schedule object itself. Be sure to do a 'Reload definition at turnaround' to revert back to the regular schedule for the next day (unless you want this task to appear again going forward...in which case you should update the Schedule object itself, too).
NOTE: I'm actually not sure how you do this in the AWI...but I may be using an old version (12.0.0) that doesn't yet support that...?
2) Instead of adding it to the schedule, just do an Execute>Once to schedule a one-time run of the task in question.