Extend vm-pool instead of adding another pool.

➕ : You don't need to juggle qubes between two pools like in the secondary storage setup.

➖ : Chances of data loss due to drive failure are higher.

Also, for encrypted setups I haven't figured out how to print what device is being unlocked during boot. It can be annoying, but it seems like system simply follows the order in /etc/crypttab. I tend to successfully guess it. Please help if you know anything about this.

1. Locate the device, partition it (if you want)

Locate the device you want to add to the pool using lsblk or other tools.

2. Add it to the pool

pvcreate $device
vgextend $vg $device
lvextend -l +100%FREE $pool

3. Profit?

This is very flexible. As you can see, almost anything can be used as the $device, be it physical device, partition, or encrypted partition. Same with the pool, you can add different amount of volume to it, not necessarily all free space available in the volume group. See man lvextend.

The algorithm for encrypted partition is a bit different, follow secondary storage : Example setup of second drive until you end up with an auto-mounted luks partition, after that continue with the step 2.