You can't access this data from either the K2 Management or K2 Client API's. You can use the reporting SmartObjects which come with K2 blackpearl to access this data however. The relevant path to the process data SmartObject is "Workflow Reports\Workflow General\Process Data" and you'd call the List method and pass in the process instance id of the process you need the data from (you can get the id from one of the other SmartObjects such as the Process Instance SmartObject.

 Hope that helps.