You can't do it in webmaster mode, you'll have to actually edit a template. The file you want to edit is customer/categories.tpl. Simply replace this:
{section name=cat_num loop=$categories}
with this:
{section name=cat_num loop=$categories}
<FONT class="CategoriesList">
Link Text here</FONT>
replace 'x' with the number of the static page you want to use, and change 'link Text Here' with the text you want to display as the link.
Make sense?