How to modify wp_list_categories?

I was asked to show the categories in the menu according to an order completely chosen by the customer: it means (as I can imagine) that the customer put a couple of numeric characters at the beginning of the cat description and then she wishes to show the description starting two chars plus to the right (using a “substr” command, perhaps). Is this possible? And where I must operate?
Of course the same is required for the function wp_list_pages, in the pages menu. Thank you for any help.