IDK if you have done this upgrade yet or not. But you're going to want to back up all of that 2tb of data before you get started.
Unless you're a linux expert, those drives don't just plug into a windows machine. The Linux ext file system doesn't work with windows.
As our numbers friend has said, the upgrade won't work that way. If you do rebuild the array twice to get the disks in there
1. that's going to take a long time and
2. you're going to have 4tb of unallocated space. I don't recall if you can or cannot use the unallocated on these units, to make another array.
Since you only have 2tb of data, you are better off doing a new array from scratch anyway.
As to growing the arrays as you increase capacity: There are other brands of units out there that do it. They're more of an enterprise level device. They're super expensive, buffalo doesn't make them either. And no, the linkstations won't do it.
Direct connecting usb drives to the linkstation unit- I wish you luck. The usb with those units has been extremely finnicky for some people. I never got drives formatted internally- to work when hooked up on a dock externally. It would always see them as blank and want to format them. Flash drives would read fine, but connect a windows formatted hard drive with data on it- it didn't like it either. It wanted to format it too.
I would not trust connecting a drive directly to the usb port to the linkstation- for any kind of backup. I do all my backups with drives connected directly to my windows pc. It's a lot less headache, and you can restore the backups on them from pretty much any machine that way. If your buffalo unit dies, your external drives formatted with that unit will only be read- by a linux os pretty much. Save yourself the headache and forget the extra port, unless you want to keep a 3rd disk online with it 24/7 and share it, or sharing a printer. That's about all the port is really good for.
And remember your raid 1 is not a backup. It is just fault tolerant to keep your data active online, and that is all it does. It still has to be backed up to an offline disk.