Hi Anna,
Assuming you're using Heading styles for your numbering, Select a par
formatted with the 'Heading 1' style, then choos
Format|Style|Modify|Format|Numbering|Outline Numbered|Customize. The
click on whichever level you want the '0' to appear after (eg Level 2
and add a '0' at the end of the 'number format' box. Then clic
OK|OK|Apply to update the style.
Cheer