I am not fully up to speed on this one but i believe you neet to add the extrafields in the localization and then need to change the code to use the extra fields.
it may be quicker to change existing fields that you dont need to collect the information you do. I did this initially but i am going to change things with the latest version because it allows me to add fields to the checkout process to get the information i need.
I believe in the trial and error method but in saying this i try and backup the database first before testing a change.