Do you have the setup with only two disks or more? You should be able to hot add it I think by using mdadm.
Something like this might work but read the man pages or howtos for mdadm:
mdadm /dev/md0 -a /dev/[new-drive]
But in my opinion and your data is worthy to you, drop RAID 0, rebuild it as RAID 1 or something else if you have more than two disks so it's striped with parity. If a drive fails in RAID 0, you pretty much lost most of your data. The only thing RAID 0 does for you is performance but it's sort of like just running with one hard drive without backups, unless you back the data up which I would encourage anyone to do because RAID is not intended as a backup.
If you insist on using RAID 0, at least get yourself 4 disks and setup RAID10, or RAID1+0, however you want to call it.