You can create a software RAID disk in the Disk Utility, but there are some cons to this:
1. Its software RAID rather than hardware RAID, which isnt as reliable or fast.
2. The discs need to all be the same size.
3. While RAID creates redundancy, you're more likely to have problems with the RAID set as a whole as every external drive you add adds to the increased risk of failure.
4. You havent mentioned backing this large, distributed volume up which is basically just asking for trouble!
If you dont have the money to go out and buy a dedicated RAID box or Drobo you may be better of just using half the discs for storage, then use the rest of them for automated frequent backups (i recommend Carbon Copy Cloner)