I understand your intention is (correct me if I am wrong) to pre-populate a product with items according to some product template.
First, you need to add object ProductItemTemplate and attribute Items to object ProductTemplate of type ProductItemTemplate. These will be used to manage product and item templates.
When you need the user to start entering a new product from product header, you can do it by a process like this:
FIND ProductTemplate WHERE ProductTemplate.Group = ProductHeader.Group
ENTER NEW Product WITH Product.Template = ProductTemplate
You also need to add a rule to object Product that will pre-populate its items based on template items, for example:
If Product.Items IS UNDEFINED AND Product.Template IS DEFINED Then
FIND ProductItemTemplate WHERE ProductItemTemplate IN Product.Template.Items
CREATE ProductItem FOR EACH ProductItemTemplate WITH ProductItem.Product = Product, ProductItem.SomeAttribute = ProductItemTemplate.SomeAttribute, ...