Increase minimal product quantity for order

[Paul Albers]

Hello,
I have some products which have minimal order quantity and above that a minimal amount of increase value.
Let's say, a product can only be ordered with a minimal quantity of 50 pieces. But when you want to order some more, the minimum order must increase with 25 pieces. Or 50 pieces.
So far I have edited some files in prestashop and added 2 columns in the database. Added increase_quantity int NOT NULL DEFAULT 1 to the tables ps_product and ps_product_shop.
If I display the var in the product template (eg {$product.increase_quantity}), it shows nicely the value I entered in the db. But the template won't accept this value because it doesn't know it and the values are not assigned, But I noticed that the up and down buttons are assigned by javascript. And there comes my problem... Where and how can I edit the files and assign the values to it?

Thanks!
Paul