looking at the code you would need to something like this.

Haxe
function scale( nine: nineSlice, s: Float ) scaleXY( nine, s );
function scaleXY( nine: nineSlice, scaleX: Float, scaleY: Float ){
  nine.width = nine.width*scaleX; 
  nine,height = nine.height*scaleY;
  nine.slice( nine.sliceTop*scaleY
            , nine.sliceLeft*scaleX
            , nine.sliceBottom*scaleY
            , nine.sliceRight*scaleX );
  // contentDirty
  // computeContent();  should be automatic..
}