On a Windows Mobile Pocket PC (touch screen): Tap+hold your stylus on a category name, in either List view or Icon view, and pick New Subcategory from the menu.

You can also make an existing category into a subcategory by dragging it under another category, or cutting then pasting it under another category.

On a Windows Mobile Smartphone (no touch screen): Select the category you wish to add a subcategory to, and choose New->Subcategory from the Menu soft key menu.