I assume you are using the SourceCode.Workflow.Client Api?

http://codecodecode.ninja/2015/06/using-k2-worklistcriteria/

If so, it does not look like there is a 'WCField' enum for Process Originator.  But this Originator object ('User') does seem to be accessible from the 'SourceCode.Workflow.Client.WorklistItem' class:

WorklistItem.ProcessInstance.Originator

Perhaps this can be applied on the result set that the WorklistCriteria would return.