You may find that you would like to add a device to be monitored by OnSight which has not been automatically discovered (for any number of reasons). In this case, you can add the device to your control panel as you would any other server. The only adjustment you will need to make is to change the monitoring location to your OnSight. All of your OnSights will be listed under OnSight Appliance in the Primary Monitoring Location dropdown of the configuration drawer.
Once you select the correct OnSight, and create the server, you can start setting up network services checks on this device.