Based upon your question, it sounds like you are having users configure an individual username/password account on the NAS, as well as on ACS.
ACS does not have any functionality to change the local password on the router (and is not even aware of it). Generally, the local username/password on the router is a generic, system administrator account. Most administrators don't allow many individual user accounts on the NAS because it doesn't scale well and is difficult to manage.
You can use your network management software (CW2K or whatever you use) to rotate the local NAS accounts.
HTH
Jeff