On ubuntu, it's possible to use a derived key from the root as an additional key on other filesystems. This has the benefit of keeping your key for the other drives out of the filesystem itself.
Before doing this, first off, make sure /tmp is mounted on ram only! I suggest single-user mode for this change.
mount -t ramfs none /tmp
Then, you can export the derived key:
# replace vda5_crypt with the cryptsetup name of your root luks
# have a look in /dev/mapper or 'pvdisplay' to find it...
/lib/cryptsetup/scripts/decrypt_derived vda5_crypt > /tmp/key
And then add it to your other device(s):
# use your own disks here instead of sdb1 sdc1 sdd1 etc
cryptsetup luksAddKey /dev/sdb1 /tmp/key
cryptsetup luksAddKey /dev/sdc1 /tmp/key
cryptsetup luksAddKey /dev/sdd1 /tmp/key
This will enable the ubuntu init scripts to use the derived key once the root is unlocked to unlock the rest of the block devices and make them available similarly under /dev/mapper. I'm not sure if they require /etc/crypttab entries - try them without first, and if they don't appear, put them into crypttab without a key and it should unlock them.
(I haven't tested any of this.)