Yes, a user is able to add a link in more categories.
Go to Add.pm and change line 360 in which you can find.
$category = $html->get_all_categories(\@ids, 'CatLinks.CategoryID', 1);
to something like:
$category = $html->get_all_categories(\@ids, 'CatLinks.CategoryID', 5);
and in Modify.pm
change the same string on line 587.
Note, there are 2 disadvantages:
1. The payment system is build on adding a link on one category, not on multiple.
2. If you have a high amount of (sub)categories, the drop down boxes will be quite large.
Good Luck!