It sounds like you're new to your z800 so it may be worth reading some documents.
You can start by looking at the various documents located here. Oddly, one useful document, the "HP Z800 Workstation Maintenance and Services Guide", did not appear within the available docs on the linked page but google can always come to the rescue if you know what to search for; you can find that doc here.
Downloads for your z800 can be found here.
Since the z800 came with many configurations and versions, it's difficult to provide specific advise without knowing the details of your system. Doubly difficult as i don't have a z800 to reference 
Generally speaking though, when configuring a volume within a disk array, you select the HDD's that will be part of the volume. As such, the other HDD in your system containing the OS should not be impacted by the RAID volume creation.
BUT there is a warning that i must make;
If your OS drive is connected to a SATA port and the BIOS SATA emulation mode was not set to RAID+AHCI when you clean installed the OS, then if you change the BIOS SATA emulation mode to RAID+AHCI, your system may not boot and you may be looking at a blue screen. This should not be an issue if you are using the HP OS image as these images have drivers for both SATA emulation modes (RAID+AHCI & IDE as called in BIOS).
It's easy to test if the above is a concern by setting BIOS SATA emulation mode to RAID+AHCI and seeing if your system will boot. If it does not boot you can always revert the BIOS and boot again (where google or the forum can help find a solution).
If your OS drive boots when BIOS set to RAID+AHCI, then simply install your new HDD into the drive caddies and create a volume across these new HDD (but dont include your old OS HDD in this group).
The new HDD that you include in your raid volume will be wiped clean so be warned. If you have any data on these new drives that you want to keep, back it up first.
Again, as you have not include the HDD containing your OS withn this new raid volumen that you just created, all should be OK and you will be able to boot and see a new volume within your OS that you can now store your data on.
Just remember, RAID is not a substitute for a backup... The purpose of raid is to provide for data uptime.