Hi, 

So, you can do this, by creating a custom style and ControlTemplate for the RowSelectorCellControl, and placing a Tooltip inside of it:

If you want the tooltip to display the index of the row, you can do something like:

<ToolTipService.ToolTip>

                                <ToolTip Content="{Binding Path=Cell.Row.Index,RelativeSource={RelativeSource TemplatedParent}}" />

                            </ToolTipService.ToolTip>

Note, this will be the actual index of the row, which is 0 based, so you'll most likely need to write an IValueConverter to manipulate it to match your Seeding index, which by default is 1.

Hope this helps, 

-SteveZ