This is possible, it’s just a pain to find…
Settings (gear icon)…
Content dropdown (left menu)…
Private Content…
“Emails” tab…
There are four registration email templates that come up. Each has an “Actions” button underneath. Click that button and select “Create”. This opens a new tab. From there, select a template and design the email. Once that’s done and published, go back to the page mentioned above. The dropdown for each of the four registration emails will have your newly designed email as an option.
I agree that this is a pain. Hubspot’s UI for setting this up leaves a LOT to be desired, but at least it’s possible.