From OpenNMS
Collect Data from Cisco Netflow using SNMP
You can setup netflow on cisco devices and collect data from it using SNMP, see
http://www.cisco.com/en/US/docs/ios/netflow/configuration/guide/cfg_snmp_mib_mon_nf_ps6922_TSD_Products_Configuration_Guide_Chapter.html
If someone has realized this using OpenNMS please add here how to configure this.