It turns out you don't need to manually add storage to the cloud tier cache. When you configure cloud tier on the DDVE, it automagically adds storage to the cache. Hopefully, this will save someone else spending hours trying to find the undocumented answer.
I have a question about implementing cloud tier on the IDPA  The IDPA comes with a 5TB Cloud Tier license and I’m trying to set it up using AWS as the target. In order to enable cloud tier on the IDPA though you have to assign disk space on the DDVE for the cloud tier cache. The IDPA documentation only covers basic installation and operation and refers to the Avamar and Data Domain documentation for anything else. However, when it comes to adding disk space to the DDVE, it is not the same, since the DDVE on the IDPA is configured as a vApp and warns you not to alter it directly. My question is, what is the correct procedure for adding disk space to the cloud tier cache on the IDPA?
IDPA Cloud Tier Configuration