Hi,
Because of that function category_select doesn't use internal class syntax (something like $this->), you just can put a copy of the whole function in your theme functions.php, the only thing you need to do is change:
static public function category_select
to something such as:
function cust_category_select
Then you can adapt it to your needs, and replace in item-post.php and item-edit.php:
<?php ItemForm::category_select(null, null, __('Select a category', 'realestate')); ?>
to
<?php cust_category_select(null, null, __('Select a category', 'realestate')); ?>
Regards