It depends on what you want to achieve.
\XLite\View\TopCategories is the widget that renders the Categories menu. Depending on the menu mode, it uses different templates from the "[xc5]/skins/default/en/categories/[mode]/" directories.
However, I guess you just want add another block to the side bar. If so, you should create either a new template, or a new widget (the PHP class) and add the
Code:
@ListChild (list="sidebar.first", zone="customer", weight="100")
tag to comments to the file (weight determines the position of the block).
You can read more about the @ListChild tag here:
https://x-cart.atlassian.net/wiki/display/XDD/Templater+lists
"sidebar.first" is the "list" for the left side bar.