If NTA receives viable packets from an IP monitored with NPM then it should just automatically add them to the list. Any chance you could throw wireshark on your orion server and confirm that it is seeing the netflow packets from the ASR? If you are getting them, does it show as coming from the same host IP as the one you use to monitor the router? If it was an unmonitored interface there would be an alert near the top of the menu bar indicating that it was getting flows from unmonitored interfaces and you would need to add them.
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