I don't think you're going to find a magic bullet here.
Does NNMi monitor every single IP in your network? Probably not, right? So how would it be able to tell which IP addresses are in use?
If you're looking to monitor DHCP scope usage, then you're better off monitoring your DHCP server.
If you're looking at usage of static IP addresses, then you're probably better off using a proper IP address management tool - it should give you all of this data.