Generally speaking, you cannot change the "type" of control for html objects. If it is a <span> control, then we interpret the control to be a <span> object and nothing else. There is probably a framework IBM is using to customize the <span> or a parent object to make it a TreeView on the webpage but OpenSpan will not be able to determine that. It could be that you need to identify the html events to invoke on the tag or you need to invoke the methods on a different control in the hierarchy. It is best to review the html directly to get a better understanding of how the webpage works. Then design your automation to do the same actions.
For example... when a user clicks the expand icon, the browser (I'm guessing) sends the "mousedown", "mouseup", and "click" events on the control. So you should try the same in automation with the RaiseEvent(eventname) method.