The thing that you wish to accomplish is not a straight-forward and easy task. First of all you need to position the sliding zone to the right. Here is a sample CSS for that:
Then you should update the position of the expanding sliding pane in the OnClientExpanded event handler:
function updatePanePosition(pane, args)
var element = pane.get_element();
Finally, in order to make the expanded pane align with the splitter, your best option is to set the position of the splitter to relative:
Please, note that there may be cases when this may not work 100% as this is a custom solution.
Hope this helps.
the Telerik team