I don't know anything about postnuke (or any other pre-made package)... I build my own stuff. But, this is pretty simple to do... and would be a good way for you to learn PHP.
Simply setup a db to hold your data, setup one script that takes input from a form and inserts it into the db (you'll want to make people login to this). Then write some code for your site that grabs the data out of the db and displays it.