Are you currently using MAC trac?
Some devices can be setup by the administrator with no expiry. Those would be specific administration devices that would be more permanent. You might want to have a class of user that is capable of setting up permanent devices besides the systems operators of clearpass (you don't want just any user to be able to do this).
You can also set the endpoint database cleanup to zero, so things are never cleaned up. When you are ready to do an actual cleanup, you can set the number to whatever you want, and devices older than the number will be removed the next morning. That is probably the cleanest way to do it.
Others might advise you differently, so I would solicit the advice of a ClearPass expert that is aware of the details of your setup, your system and your IT processes.