By two fields are you talking about two varbinds?
The trap definitions in the netapp.mib state that each message will be comprised of two varbinds:
"OBJECTS {productTrapData, productSerialNum}"
In my experience productTrapData contains the message payload about the alerting event and productSerialNum contains the serial number of the Filer.
To the best of my knowledge the varbind structure cannot be changed but I'm not an SNMP guru.
The only customization you can do it create your own traps but the basic object structure of the trap needs to follow the structure outlined above (again to the best of my knowledge).
Richard