You can try a server event with this the relevant line of code.

Examples.

K2.ProcessInstance.Logger.LogInfoMessage("My Process", "My Info Message");
K2.ProcessInstance.Logger.LogErrorMessage("My Process", "My Error Message");
K2.ProcessInstance.Logger.LogDebugMessage("My Process", "My Debug Message");
K2.ProcessInstance.Logger.LogWarningMessage("My Process", "My Warning Message");

The other codeless way is that you can use the Logger Event wizard on the K2 underground.

http://www.k2underground.com/groups/k2_logger_event_wizard/default.aspx