I could not find this elsewhere, so after I have found a solution for my problem, I would like to post it here so other users may benefit from it.

My problem was that I would like to get one of the categories in the left side to link to another place than the standard category-list.

To do this, change the following in /catalog/includes/boxes/categories.php

CODE
$categories_string .= ' href="'.tep_href_link(FILENAME_DEFAULT, $cPath_new);


to

CODE
if ($tree[$counter]['path'] == 26) {
$categories_string .= ' href="'.'your new link path here';
} else {
$categories_string .= ' href="'.tep_href_link(FILENAME_DEFAULT, $cPath_new);
}


where 26 here is an example of the .../index.php?cPath=26

Then you can do this for every path number that you wish, just add another 'else if'-sentence and another category path number.