Howdy,
That’s a tricky one to automate, unfortunately, due to the large number of possible setups folks can have.
You don’t need to use RAID or LVM to add a disk… but if you were already using them, the procedure for doing this would be different.
As you mentioned though, it’s not really possible to begin using RAID or LVM after the fact though, that’s something you’d need to use from the beginning when your OS is initially installed.
If you have an extra drive, and aren’t using LVM or RAID – that means that drive would be used just for the files where you mount it. The space would not be combined with your existing drive, the space would just be available for one particular mount point.
I might suggest using it for /home, for example (/home is the mountpoint).
You’d first need to format your spare drive, partition it, mount it somewhere temporarily, copy all your files to it, unmount it, mount it overtop of /home, and then add an entry for that in /etc/fstab.
So, there’s a lot of steps involved in accomplishing this
It can all be done from the command line, and that’s probably the quickest way to do it.
If you aren’t familiar with the command line procedure for those tasks, you should also be able to do all that from within Webmin.
You can partition (and format, I believe) in Webmin -> Hardware -> Partitions on local disks.
You can mount a drive using Webmin -> System -> Disk and Network Filesystems.
Once you’ve mounted your new drive on a temporary location, you can use a file manager to copy the current contents of your /home to that temporary mountpoint.
That should hopefully get you started
-Eric