How can I add a 9-slice to an NGUI Progress Bar foreground sprite?

Is it possible to add a 9-scale slicing to an NGUI Progress Bar foreground sprite?

The reason is because I have a horizontal foreground metre that has rounded ends and would be best if I could leverage the 9-slice technique by setting the borders so that when the foreground sprite extends (as in the progress bar progresses), it extends from the middle part of the 9 slices, but not cut away the ending round part of the foreground sprite.