That is correct, but you would only see the space created in Disk Mgmt, it will appear as Unallocated Space.    
Please i have Poweredge R320 server with controller 405-12068 : PERC H310 Integrated RAID Controller and have RAID 1 (mirroring) with two SAS hdd 278.88GB. I want to add sata hdd ( Dell W8FW5 Seagate Enterprise Capacity ST2000NM0018) to be data disk to arhive files. Is it possible to do that but not to reconfigure RAID settings. Thank you so much.    
Dell Poweredge R320 add SATA hdd Dell W8FW5 Seagate Enterprise Capacity 
ST2000NM0018