@emilio: Here's what I did to get the product_extra_fields on products that were in my catalog before I applied the PEF add-on:
NOTE: Backup your database before you do this stuff (in case it goes wrong).
Start phpMyAdmin. If you've installed PEF correctly, you'll see a product_extra_fields table within your zen-cart database.
If that's there, go to your product table and write down the products_id values for all the products that you want to have the extra fields. Now, click on the product_extra_fields table name on the left-side of the display. Up at the top of the screen, click the *Insert *tab and you're presented with an entry screen to create a new entry in the PEF table.
For each of the products_id values that you want to have the extra fields: enter the products_id number in the products_id *Value *element of the form, enter in the extra_field Value element of the form, and keep the *Null *checkbox checked for all other elements. Click "Go"; that will add an element in the PEF table for that products_id.
Note: If you have a bunch of products_id's to add, make sure to look at the bottom of the page and select "Insert another new row" as the action to take after the current row is added.
It's cumbersome, but it worked for me!