You don't need to, that's done already for you automatically in the Allocation phase. That's the whole point of reservations is so you don't have to manually create logic to determine capacity.
That's not how vRA works and is antithesis to the whole idea of cloud provisioning. Reservations exist to be capacity gates. When you automatically increase a reservation that runs out of capacity, there becomes no point in a reservation. It's like a paper wall.
There's probably a way, but the EBS cannot do that because the allocation happens before EBS runs. But what I'm saying regardless is it's not a good practice and you shouldn't do it. If a user's request fails because of reservation capacity, then there needs to be a business discussion around why reservations are configured the way they are and if an additional purchase of resources is necessary. The answer shouldn't be to just blindly increase the reservation. Again, if you're trying to do this, then you're using vRA in the wrong way.
