The Workspace reports use out of the box SmartObjects for their data.  These SmartObjects can be used in custom code also.  I have worked up a simple example here to show how it can be done: 

http://k2underground.com/blogs/fromthebench/default.aspx

HTH. 

THX  Bob, it's very helpful.

Now i can get the process information I needed.

But  is this the only way?

Is there any K2 API I can use?

The SmartObject API is the only way this data can be surfaced.  The other commonly used K2 APIs, SourceCode.Workflow.Client and SourceCode.Workflow.Management, only deal with active process instances.