How to add a new LUKS partition and open it with the global passphrase.

[yudi v]

I have a system setup with LUKS on top of LVM, I plan to add a new LUKS
logical volume.
Adding the entry for this LV into */etc/crypttab,*  makes it ask for the
passphrase at boot.
Now, what I want to know is how to make this partition decrypt using the
existing "global passphrase".