David is right. Build the RAID as you should, then create a filesystem on the disks, open /etc/fstab and comment out the existing /home entry. Add the new /home space to the fstab file, adapt the device path to suit your system.
To copy the old /home to the new /home, uncomment the old /home entry (in /etc/fstab), make sure to change it's path (like /home/oldhome) but create the new directory beforehand. Copy the files to their new home (
), make sure you've got them all (there are several hidden files, all starting with a dot) and when you're done, remove the entry in /etc/fstab to the old /home.