I've just checked a number of my trunk links on my Cisco switches and all show the information you are requesting. So it should just work by default.
What version of NPM are you using? And how is your trunk link configured.
superfly99 this is how my Trunk port is configured
The interface is part of a port channel
Switch_1#show running-config interface te1/1/1
channel-group 1 mode on
sflow 1 sampling 1024 256
sflow 1 polling 256
Switch_1#show running-config interface port-channel 1
switchport mode trunk
switchport trunk allowed vlan 1-2,120,130,140,150,160,170,180
You are correct, when the interface is part of a port channel, you won't see the VLAN info unless you look at the port channel itself. Then you'll see the Assigned VLAN's.
That's a real pain when you are doing a discovery with hundreds of ports, and you can't use the available filters, or a regex, to group your port channels and their members!
They are all trunks, but the port channel members always get mixed in with the general Ethernet ports.