Online expansion feature is very expensive and risky. So my suggestion is back up and recreate a new volume then copy data back...
Here is what you need:
Get a low power MB like ATOM - This will save the electric bill
Or use the door stopper system just sit in the corner.
2GB of RAM
1x USB 1GB or larger
1x Mini Tower can hold 5x HDD - you can use 5.25 to 3.5 bracket to install 3.5 HDD to 5.25 slot too this cost about $2.00 or less each
1x GbE if the old computer does not have GbE - ideally Intel NIC would be great
SPM394 <-- you can see what going on with the RAID - this is I prefer
or SPM393-I <-- you need to connect all the LEDs indicator pins so you can see what going on
Download FreeNAS ISO file
Burn to CD
Connect temporarily a CD/DVD rom to the NAS mother board
Boot from CD
Install OS to USB stick
Remove CD/DVD rom after installed OS
Connect 5x HDD to SPM39x
Configure RAID5 or RAID5+HS. This takes about 30~45sec
Connect to your network
Reboot the system with USB
System should boot and give you the IP address
At you computer - open a web broswer
Enter the IP address the FreeNAS just assigned
Log in as: admin - password: freenas
Format and mount the RAID
Enable services that you need: SMB, FTP, iTUNE
Make a share folder
That's it
Within 1hrs you should have a NAS ready to stream ANY Audio/Video
Oppsss! Well not that fast...
Sorry to burst you bubble
You need to load you DVD/CD, MP3... 1st
Which is VERY easy
Open Window Explorer
Enter: \\IP address - the network drive should show up.
Map this drive
Now you can load your content
If you DO NOT want to go through all of DIY
Get this
NAS
It's pretry much what I list it here... except they're tray-less and have fan on each drives so the HDD life is prolong