Is this still an outstanding issue for you.
To the best of my knowledge, you would create the readonly account in SNMP within your network mgt utility.
On the PANW FW, you are merely creating an record/config that will use the snmp account name created on the snmp application.
there is no ability to create a local snmpv3 account on the FW. We are sending traps TO the snmp app.
Unless I misunderstood your query.
... View more