Welcome to NethServer! Its a great distro to use for a small to medium sides office to use.
If you are using hardware to define the RAID, then all you would need to do is add the hard disk, add it to the RAID set and the OS should (in theory) see the extra space. This route may require downtime while you add the disk to the RAID volume. Replacing disks in the RAID volume won’t require downtime if the RAID is configured correctly.
Hardware RAID will take some of the workload away from the CPU and give the server itself more power to work on other tasks.
If you are using software RAID, you can use SSM, please do let me know how to get SSM installed, haven’t figured that out myself yet. There are other tools to manage the LVM.
Instead of adding the slower SATA disks to the SAS LVM, one option would be to create a second LVM for the SATA disks and you can then mount that LVM volume onto a predefined folder, eg. mount it to /media/2TbLVM/.
This way, you can still retain the speed of the SAS LVM and also get the additional space that the SATA disks will provide. There is no point in paying extra for the speed of the SAS disks and not being able to use the full speed or functionality they provide.
This option would also provide a solution to your second question you raised regarding the shares.
Regarding editing the smb.conf files, I would not recommend editing it. If there is an exceptionally strong business case to make any changes, use the e-smith templates and be very very careful with the changes you make as it may break the
Shared Folders functionality.