openITCOCKPIT supports the use of Checkmk Raw Edition for many years. The first implementation was already available for openITCOCKPIT Version 2.x.
Checkmk is available through the CheckmkModule. It is possible to monitor via the Checkmk Agents or through SNMP. Services can be found by an discovery which makes it easy to add them to the monitoring.
Documentation:
Monitoring through Checkmk
Monitoring through SNMP using Checkmk