You're not going to affect the "product id" with an extra field - that is set automatically when the product is created, and you should not try to change it. If you mean the product's model number, that is something you can input when creating/editing the product, but not subject to the extra fields in this mod.

You will have to explain in more detail what number you are talking about, and Delia will have to be the one to answer, as I don't know the code for this mod.