Not a problem.
Deploy OpenNMS in your primary site and then set up our Minion appliance component at remote sites. The core server will control the Minions to monitor nodes at that site and all collected data will be rolled up from the Minion to the core OpenNMS instance for a centralized view of your network. If you're collecting flow data for multiple sites, we also have a Sentinel component that can provide horizontal scaling to offload flow processing from your core server.