I have a classifieds section on my site and can only add sub-categories to the main categories, is it possible for me to add sub-categories to sub-categories, if so, whats the code and where do I add the code?

Im a bit of a web newbie at the moment but am picking things up quite quickly, but if you could explain it in very easy to read format that wud be good.

The category files for the site are here:

http://files-upload.com/files/614283/admin_category.pl

http://files-upload.com/files/614284...subcategory.pl

Any help appreciated, thanks.