luckily you have NTA 3.10 :-), there is no need to have SNMP enabled or even pingable machine in order to receive NetFlow from it (if you are ok with specifying interfaces names and speed manually). To do that, first monitor the device as ICMP node (or external if permanent red status bothers you) in Orion. Then you should be able to see Event in NTA telling you that you are receiving flows from unmanagable interface or something like that, with a link to add the interface. The link will send you on the page where you will set a name a speed of that interface and submit. That's it, now you should be able to see NetFlow from that device in a couple of minutes.
If an event would not appear, make sure you are really receiving flows on Orion machine (using wireshark).