Create your own drawable, for example progressbar.xml and set android:progressDrawable="@drawable/progressbar attrubute on your progress bar
write in progressbar.xml drawable this code:
<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
<item android:id="@android:id/background">
<nine-patch android:src="@drawable/progressbar_bg" android:dither="true" />
</item>
<item android:id="@android:id/progress">
<clip>
<nine-patch android:src="@drawable/progressbar_body" android:dither="true" />
</clip>
</item>
</layer-list>
and replace @drawable/progressbar_bg and @drawable/progressbar_body with you own 9 patch png images