I think you can just do this (someone correct me if I am wrong):
- Partition second disk however you want
- As root, back up /etc/fstab
cp /etc/fstab /etc/fstab.BAK
- Then edit fstab as root
nano /etc/fstab
Mine looks something like this, yours will be different but the important part is the area where it marks the partition as /home. I suggest trying this in a VM first until you are comfortable enough to do it on the host. You just need to know the name of each partition (which you should be able to find out from something like gparted)
/dev/disk/by-id/ata-KINGSTON_SH103S3120G_50026B722A0480DC-part2 swap swap defaults 0 0
UUID=bc9a8b77-7e43-44b1-9d57-95b15dfff6b3 / btrfs defaults 0 0
/dev/disk/by-id/ata-KINGSTON_SH103S3120G_50026B722A0480DC-part4 /home xfs defaults 1 2
Then once you have gotten it to work, just mount the partition that used to be /home and copy over the data you need.