You can add additional dates and times to a recurring volunteer opportunity from the list of individual opportunities.
Click the icon beside the recurring volunteer opportunity, then click the link to view all opportunities.
At the top right of the list of opportunities, you'll see a button to add a new opportunity.
Click this, and fill out the form.
NOTE: If after adding an off-schedule opportunity, you modify the parent recurring volunteer opportunity and change the schedule, the off-schedule opportunity may be removed.