If you have a schedule already in place, you can extend it by manually adding shifts to the schedule. Just make sure that this does not cause the schedule to overlap with any others. 

1. Click on the day that you want to add shifts
2. Click "New Shift..."
3. Put in the time and specify the desired number of volunteers 
4. If this is a regular daily shift or a recurring monthly or yearly shift, go to the "Treat as:" section and choose the appropriate shift from the drop-down menu
5. Click "OK" to create the shift and extend the schedule

Once you have all of the shifts in place, you can run the auto-scheduler for just those dates by:

1. Click on "Commence Scheduling..."
2. Choose "Show more options..."
3. Choose "All empty positions in shifts I select or during a time period of my choosing..."
4. Set the time period for the dates you just added
5. Click "Commence Scheduling" to start the auto-scheduler