Eddie,

You can make any number scheme your defualt if you set it up in the document
template.

Let me assume that your statement "the option is there" means that you see
it on the Outline Numbering tab accessed through the menu comand or Bullets
and Numbering toolbar icon.  If I am correct then you are headed for shoal
water and let me offer a course change.

If you are serious about numbering documents, do not use either the
Format>Bullets and Numbering menu command or toolbar icon to set up your
document numbering.

Shauna Kelly offers step by step instructions for laying out outline
numbering here:

http://www.shaunakelly.com/word/numbering/OutlineNumbering.html