First off, SNMP Traps don't have a "Severity" field. Severity can be found in Syslog messages but not in SNMP Traps.
I have checked Orion NPM Trap Viewer Tool and yes it can forward SNMP Traps, however it can't parse or change the Trap message and add Severity field.
With that, maybe you can try considering using Solarwinds Kiwi Syslog Server application. This is dedicated Syslog Server that has the capability to accept SNMP Trap message, add Severity (Level and Priority) Fields to it then forward it to your Zenoss device.
Hope it helps.
Oh.k i thought there could be some option to configure since its a very basic thing.
A new trap template also wont help? I read somewhere that we can write our own trap templates.