Look in the administrator's guide. There is a way of enabling the local Casper management account on the Mac for FileVault 2, but keep in mind, it means that account name shows up at the initial FV2 login screen later, along with any other enabled accounts, like the primary user for example.
Here's a link to the online documentation that offers more details on that:
http://docs.jamf.com/9.96/casper-suite/administrator-guide/Administering_the_Management_Account.html
In case you were wondering, there's no way of automatically enabling another local admin account that isn't the Casper management account, at least not without using some type of user interaction process that would ask the current FV2 enabled account holder for their password so it can be used to enable another account for FileVault. There are some custom scripts posted from users here on JN that do this, if you search around for them.
