Solved it like this:
string query = "Select SI_DEST_SCHEDULEOPTIONS, SI_PROGID From CI_SYSTEMOBJECTS "
+ "Where SI_PARENTID = 29 AND SI_NAME = 'CrystalEnterprise.DiskUnmanaged'";
InfoObjects destinationInfoObjects = infoStore.Query(query);
InfoObject infoObject = destinationInfoObjects[1];
DestinationPlugin destinationPlugin = (DestinationPlugin)infoObject;
DiskUnmanaged diskUnmanaged = (DiskUnmanaged)destinationPlugin;
DestinationOptions destinationOptions = diskUnmanaged.ScheduleOptions;
DiskUnmanagedOptions diskUnmanagedOptions = new DiskUnmanagedOptions(destinationOptions);
diskUnmanagedOptions.DestinationFiles.Add(filename);
diskUnmanagedOptions.UserName = SMC_Reportserver_Classes.Properties.Settings.Default.printFileUser;
diskUnmanagedOptions.Password = SMC_Reportserver_Classes.Properties.Settings.Default.printFilePassword;
schedulingInfo.Destination.SetFromPlugin(diskUnmanaged);
InfoObjects infoObjects = report.ParentInfoObjects;
infoStore.Schedule(infoObjects);