Regarding accessing the custom fields created in product module to the Invoice module

I have created the products in the product module. I have added additional fields in product as warranty and warranty type.
In invoices, i capture the product created , but i also want to capture the waranty and waranty type , the new fields added.

In the study , layout , i am not able to see the fields, i created in product module.

How do i access the fields in product module to Invoice module.

thanx