I know there is a conversion script for batch import of NAS devices (NAD in ClearPass) in Aruba Solution Exchange (ASE): https://ase.arubanetworks.com/solutions/id/95, and it is rumoured to take the ACS export format directly; but please verify that.
Then there is https://ase.arubanetworks.com/solutions/id/94 which can import users into the Local User Database.
Also there is an import for endpoints, which fills the ClearPass Endpoint DB, at https://ase.arubanetworks.com/solutions/id/91 however, I don't think ACS has such a database; so nothing to import.
Import formats are provided in the solution, and if you are asked to login, you can use your Airheads account for access to ASE.
BTW: I have not seen ClearPass 6.7 yet; you probably mean ClearPass 6.6.1.