As far as I know, it can't be done.
You can create an encrypted CoreStorage volume on another disk/partition, but if you want full FileVault 2 functionality, it must be a bootable volume. You'd then boot into that volume and use the normal methods to enable FileVault on the startup disk.
Greg's right, this isn't possible currently with a non-boot disk. You can set up the non-boot drive to be encrypted, but it can only be unlocked with a password.
I have a couple of posts on how to encrypt a non-boot drive available here:
If you need non-boot drives to be encrypted and have alternate recovery methods, I recommend looking to hardware encryption. IronKey has solutions that may work for your environment: