I have found that the value in the source field of an alarm varies depending on the probe. For example, the interface_traffic probe can actually be configured to use the profile name, description, or IP address as the alarm source. If your NAS can resolve those IP addresses back to names, the hostname field in the alarms should be just what you need. However, if the NAS cannot resolve the IP addresses, sometimes the hostname field will also just contain the IP address.
-Keith