In Discovery wizard you can schedule the discovery to run regularly, there is an option for custom interval. Afterwards the results of newly added servers are visible on Network sonar discovery's "Scheduled discovery results", there you can import only new servers.
Hope that answers your question :-)
Do you use IPAM?
In addition or besides a discovery you could do a simple report comparing NPM and IPAM where IP_Address = Used. This would really only be helpful if you trusted people to fill in custom properties in IPAM such as Status=Production then you go off those.
Scheduled Discovery does provide more actions after discovery though and an ignore list.
Thanks for the feedback both, i dont use IPAM but will look into it. Pavol i will see if i can get the discovery to work.