You can always add individual features to your phone system. It doesn't matter if you're on the Pay As You Go plan or the Per Seat Unlimited plan. If you would like to add a feature that is currently not included in your Per Seat Unlimited plan, simply log into your admin portal and click to add features through the Apps or Resources tabs.
Let's say your business is on the Per Seat Unlimited Plan. You have 15 users each with their own personal conference suites but you need another one for a long-term client of yours. To add another conference suite, simply create a new one through the "Apps" tab.
You can see this extra application added to your future invoice by going to the "Accounts" tab and clicking on the "Configure plan" link. In the "Configure Plan" box, you will see your new conference suite under "Additional Resources", and the monthly total that you owe will increase accordingly. Any additional features are priced as Pay As You Go features.