I don't think it is with existent styles. To get this to work you would have to override the updateDisplayList method and draw the top border using vector graphics.
override protected function updateDisplayList (w, h):void
super.updateDisplayList (w, h);
graphics.lineTo(w , 0);
If you are using Flex 3, set the borderSides preoprty to ['top']. Check the following example. It uses a NumericStepper but you can achieve the same thing with HBox
If you are using Flex 4, Check the following cookbook post on basic skinning
Thanks for your reply it helped me to solve the problem.
Thanks & Regards,