Unlimited Sandbox for Existing Production Projects

Greetings.
From my understanding, Sandbox developer accounts usually last about 30 days and can optionally be extended via manual requests. This is highly inconvenient for projects where the there is a deployed dev environment, apart from the production environment.
My project already has a production environment that the owner is paying for. However, as an administrator, I was unable to allow testnet tokens in the mainnet vault, nor switch to a parallel Sandbox workspace - I saw no buttons for this.

The documentation doesn’t seem to mention this case. What is the procedure?