Thanks for you reply. Probably expressed myself wrong. I would like to simply enter order id and form value (form is on carrier page) to table (only includes: id, extra). I checked carrier file but guess I should look for a place where orders are going to be submitted to SQL?
Found thread: http://www.prestashop.com/forums/topic/84550-solved-where-is-the-insert-into-statement/
It's a bit similar to what I want to do, but at the moment code lines aren't same any more.