If you would like users to be able to make a standby schedule (i.e. a schedule that will become active if the primary schedule is cancelled), then you can enable standby schedules.
The following article explains how standby scheduling works:
How to enable standby scheduling
To enable standby scheduling, go to Group Info >> Sched Policies to enable and configure standby scheduling. The figure below has a screen shot. The bottom section has the standby rule configuration. Check the box labeled 'Enable Standby Schedules'. Review the defaults for when a standby must be confirmed and when it will be cancelled. It is usually best to start with the default parameters until you have a clear understanding of how this feature works. Do not forget to click 'Save' when done.