You should make use of Paragraph.setDepth() propert in JAVA or Paragraph.Depth in .NET.
Please see the JAVA code below and its ouput presentation.
JAVA
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public static void IndentedNumberedBullets() throws Exception
{
Presentation pres = new Presentation();
Slide sld = pres.getSlideByPosition(1);
com.aspose.slides.Rectangle rect = sld.getShapes().addRectangle(100, 100, 3000, 2000);
rect.getLineFormat().setShowLines(false);
TextFrame tf = rect.addTextFrame(" ");
Paragraph para = tf.getParagraphs().get(0);
para.setHasBullet((short)1);
para.setBulletType(BulletType.NUMBERED);
para.setNumberedBulletStyle(NumberedBulletStyle.BULLET_ARABIC_PERIOD);
para.setNumberedBulletStartWith((short)1);
para.setText("Example paragraph 1");
Paragraph newPara = new Paragraph(para);
newPara.setDepth((short)1);
newPara.setBulletOffset((short)100);
newPara.setTextOffset((short)100);
newPara.setText("Example sub paragraph 1");
tf.getParagraphs().add(newPara);
//Clone the previous paragraph
newPara = new Paragraph(newPara);
newPara.setText("Example sub paragraph 2");
tf.getParagraphs().add(newPara);
pres.write(new FileOutputStream("c:\\outNumberedBullets.ppt"));
}