Best way to automatically create and add a product to purchase.order.line ?

Hi ! I'm creating a shipping module for purchase order. Now that i've created the config part, I want to create and add automatically a line to a po when the po is generated, either by hand, or by purchase requisition. This line must be a product, with description and price based on the config part and the supplier who will receive the po. My idea is to create a function to do the stuff, but I don't know where to call this function, and if it's possible to create and add the "shipping" product. I know that's an unusual question, but thanks for guiding me.