you seem to have a strange requirement. i would recommend to make a custom version of xowiki/www/portlets/categories.tcl with a different name and add the following two lines after the lappend categories $category_id (around line 46)
set itemobj [Object new -set name en:index -set title MyTitle -set prefix "" -set suffix ""]
$cattree(0) add_to_category -category $c -itemobj $itemobj -orderby title
this adds for each category an entry en:index with the specified name and title into the category tree.
hope this helps