Enable network user accounts to unlock encrypted Mac (OpenDirectory/FileVault)

It depends on your definition of a network user.

At the pre-boot stage that is running when a FileVault2 login is occurring the Mac has no network connection running at all and you cannot use a full blown network account because without a network connection it will not be able to contact the directory server. Furthermore the pre-boot login is only allowing accounts that have been explicitly allowed and this is achieved by having their credentials stored in the pre-boot login environment which obviously is not part of the main boot environment since the main boot environment is at that point stored on the encrypted drive and not accessible.

What is possible is to create a 'mobile' account. This is a local user account which is linked to and synchronised with a directory service such as Open Directory or Active Directory. With a mobile account you have a matching local account that can be used even when you have no network connection or are for any other reason e.g. being out of the office and unable to communicate with the Directory Server. Unless you laboriously created mobile accounts to mirror each and every single network user account only those you have added as mobile accounts and enabled for FileVault2 logins would be able to login for FileVault2 purposes.

The creation of a 'mobile' account is triggered when you login to a Mac using a network login account and the computer has been told to then automatically or offer to create a mobile account. (Often the default behaviour.)

Yes, you should be able to do this. Multiple users can unlock FileVault at the PreBoot when you first turn the mac on.

If you go int system preferences > Security & Privacy > FileVault

You should see a button that says "Enable Users" Inside that should be your directory account.

If not you can still enable the user in terminal using.

sudo fdesetup add -user directoryuserhere -usertoadd adminuserhere

Then check by running

sudo fdesetup list