I have a scenario in my web application where I need possibility to sort values in RadPivotGrid by field existing in datasource, but is not displayed in RadPivotGrid.

For Example I have flats that should be sorted in order defined in field sort_order, I can add this field to RadPivotGrid, but I don't want to show it to user. 
This approach works perfectly with standard RadGrid, but unfortunately does not work with RadPivotGrid. Flats are sorted by their names, with is wrong from my point of view.