MAKE SURE YOU HAVE A BACKUP!
Creating a software raid 1 set with mdadm is something that needs to be done through the terminal.
Once you have added the disks to the server, you have to find out how those disks are known to the system. There are several ways of doing this:
In the terminal (on the server itself or through SSH), either do:
Each disk will be shown as /dev/sdx. And if there are multiple partitions on a disk, they will show up as /dev/sdx1 /dev/sdx2 etc…
[root@ns7 ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 900G 0 disk
├─sda1 8:1 0 1G 0 part /boot
└─sda2 8:2 0 899G 0 part
├─VolGroup-lv_root 253:0 0 895.1G 0 lvm /
└─VolGroup-lv_swap 253:1 0 3.9G 0 lvm [SWAP]
sr0 11:0 1 1024M 0 rom
In above example you see a single disk of 900GB devided in 2 partitions sda1 and sda2
As soon you add 2 disks of (lets say 1000GB) you will see, besides the existing 500GB SSD you have on sda, 2 more disks of 1000GB on sdb and sdc
With these 2 added disks you will create a RAID1 set using mdadm.
There are many tutorials online on how to create a RAID1 set using mdadm. For example: https://www.tecmint.com/create-raid1-in-linux/
Now you have to set your raidset to mount to the desired mountpoint. I would use /var/lib/nethserver since that is the main directory where all data is put.
You will have to change fstab so the system knows that /var/lib/nethserver changed to the root of the raid1 set. Also for changing and adding mountpoints, there are a lot of tutorials online, but keep in mind this is not a very easy task. Neither is maintaining a raid1 set. You have to read a lot to understand how mdadm works!.
If you decide to reinstall, you can create the RAID1 set during install and mount it as /var/lib/nethserver. Use the manual or interactive install option.