You can only do this through an OS-managed RAID setup.
Hi! I have a PowerEdge R420 with 1Tb Hard Disk (No RAID with Embedded SATA.) I got 3 additional 500 GB Hard Disk. I intend to use all the Hard 4 Hard Disk (1TB that comes with the server and the 3 Extra ones I got) as one Drive using RAID 5 without losing the OS on my 1TB that comes with the OS. Please kindly advice. 
Embedded OS Drive with Additional Drive using RAID