Had a little more time today...
Assuming Windows OS, and you have important data to maintain.
Considering how unreliable Raid 0 is with multiple disks and considering larger disks are inherently more dangerous then disks of smaller disk capacity,I will not get into extending your present raid 0. Personally I have not used even a single disk raid 0 in many years for data, as the chance of data loss is just to high.
If you raid controller supports raid 5, you could migrate your present drive to that level, requiring at least 2 more drives, with no data loss. If you search the linked Dell site, the procedure has been discussed before
If you raid adapter does not support raid 5....
The incorrect post I gave is a fairly safe disk setup (create a raid 1, no more space but gaining raid 1 safety) for your original disk. To add more space, if you have more disk slots (total at least 4), purchase two more disks, create another raid 1 then implement Windows DFS to create a DFS Namespace which would allow both raid 1's to be seen as one directory structure.
DFS
as stated before always backup your data.
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Chernobyl disaster..a must see pictorial