If anyone ever needs this, it is possible to monitor an APC Network Management Card via NethServer (for the moment I tested it on 6.8, but I think this is completely specular on 7).
You need to install the NUT package from the Software Center (packages
nethserver-nut and optionally
Then you may go on the WebGUI, Configuration > UPS and set the following:
Leave device as "USB (Auto)". At the submit you will receive an error, but it is fine.
Open an SSH connection to NethServer, then become root and configure your APC Network Management Card as a device (let's suppose it is available at address 10.0.0.1):
db configuration setprop nut-server Device 10.0.0.1
And voilà, your UPS details will appear on the Dashboard, UPS section:
How this magic works has been explained on official documentation and
man 8 snmp-ups.