How to add additional NIC to guest VM machine, on linux KVM using virt-manager
I am not sure this question is a great fit for SF, in part because I am not sure the situation you describe fits within the topics you can ask about here.
It isn't clear from your question if you have the required rights to do what you are trying to do - are you the VM administrator, and are you able to configure/modify any parts of the setup?
You may find some use in looking into virsh, notably its
Alternatively, if you are comfortable with
vi and XML, you can
virsh edit your guest to add a NIC.
The fact you have no options to edit your guest via virt-manager would however suggest you may not have enough access/rights to do what you are trying to do.