  Yes, each volume (Virtual Disk or array) is seen as a separate "disk" by the OS.   Yes and no. Yes: You can span them into a single volume in Windows, but I would STRONGLY recommend against it. Yes: You could back up the data and create a new RAID 10 with all the disks. No: This cannot be done at the hardware/controller level - you cannot dynamically expand "nested" RAID levels (10/50/60) like you can with RAID 1/5/6. I believe you would be MUCH better off with a second larger volume.  
Current server configuration:Dell PowerEdge 510 with 12 harddisk bays on H700 PERC controller.  4 bays are already used up for OS and Application installation.  Configured with RAID 10  I need to install additional 8 harddisk into the remaining bays with RAID10 configuration to expand the server's storage capacity. Is this possible without affecting the existing 4 disks which I already have OS and Application and Data in it?  My IT informed me that there may be a risk when the RAID arrays are being rebuild, but it doesn't really make sense to me.  Can someone please help to verfiy?
Adding Physical Drive