What do you mean by add and how should it be used? This is normally not a PVE issue, but a general network one in which the setup is done in VMs.
First, you need upstream support for this in such a way that all IPs in the subnet get routed to your network gateway host. This could be a VM with an IP in your "normal" network and one from the subnet on another interface. This can be done via vlans (the easiest way) or via a new bridge in PVE.