From OpenNMS

For some who desire more control, or who only have small network, individual nodes can be manually added. There are two significant ways to add nodes manually:

1. Via the web-interface, by entering the IP address, or
2. Using the send-event.pl script to send a New Suspect event

In the second case for example, custom scripts can be written to obtain appropriate IP addresses from existing systems or databases, and feed these into OpenNMS