How to apply Retention Policies on Microsoft Exchange for the mailboxes provisioned by CA IDM?
The only way to achieve this is to create a Policy Xpress that will call an external code (a PowerShell script) to run the needed commands to set the policy and other settings that do not appear in the account template screens. You will set the Policy Xpress to call the script adding the needed values as parameters (mailbox name, retention policy name, etc.)
This URL has instructions on how to use PowerShell to apply a retention policy: https://technet.microsoft.com/en-us/library/dd298052(v=exchg.150).aspx