You can unmount root. You would not be able to run anything, because nothing would be avaliable. Boot you system with the install CD and do a system rescue. Don't mount you disk filesystem. The boot cd will create a ram disk with some basic tools.
This is a pain in the butt. I spent a week once trying to do this. The easiest think is to reinstall the system as a raided system. I don't know it 7.3 can do that (I think it can), but I know 8.0 does. If you want to try to convert you existing system to raid because you want to learn then this link might help.