we are thinking of using IPAM to help us manage our public NAT IP's. Is there a way to add a user defined field that we could enter the public address, while having IPAM scan the internal address?
Also our business rules say that the NATs are only valid for 45 days and need to be re-certified, and if not used in 90 days are no longer valid. Is there a way to have IPAM send an alert that the address hasn't been used for 90 days?
you may use custom fields and use it as place where you can keep your public address (this won't have an impact on target scanning address).
And yes, you may use "Last Response" field and set an alert that will notify you if date is older than 90 days. I'd recommend to download IMAP Eval and try to configure alerts via your alert manager.