How to change from passphrase to encrypted key file in debian using luks?

Question

I have four partitions plus /boot, each one with a different passphrase.

I would like to manage unlock of all partitions with a single passphrase that decrypts the gpg.key file on the boot partition.

Perhaps this would be the approach, but I'm lacking info on some point:

1. Add key.gpg file located on boot partition, to all 4 luks partitions
2. Encrypt that key file with a passphrase
3. Tell initrd that to mount root partition it has to ask that passphrase

It would be nice to do it on a existing system.

Thank you in advance,