I don't have actual experience with software raid, but I don't think that'll be possible. Also, I've heard only bad things wrt software raid in general. I'd really recommend looking into a hardware ...

I have a Mac Mini that I'm using for email services (Kerio) and want to upgrade it to Mountain Lion. Currently, it is running 10.6.8. It has two internal hard disks which I want to setup as a RAID1 ...