I figured it out. If anyone else wants to know:
* Photopost classifieds already provides 6 extra custom fields which can be allocated when editing categories in the admin panel.
* in MYSQL see pp_categories and pp_products tables. You will see rows extra1-extra6. I needed 30 extra custom fields altogether, so I added rows extra7-extra30 to both tables.
* I searched for every occurance of "extra6" and added in extra7-extra30 in the following files: admin-cat.php, showproduct.php, uploadproduct.php, editproduct.php.....I also had to adjust the templates for each of these pages, to hold the extra fields.