Since your device only support radius, you can use a Network Access Profile to match some set of conditions and apply some set of rules, for example, you can use the NAS-IP-Address attribute to match the ip address of this host from the Access-Request packet and tell that if this device comes to authenticate then authenticate to the local ACS DB, all other queries should be parsed normally. Take a look at the configuraiton guide of ACS particularly on the NAP section:
