I will assume you only want one (and only one) inventory record for each product record:
Establish a relationship from the Product table that allows creation of records in the inventory table - use the product code as the link.
Create a script to loop through your products, where the product is active, set the inventory values using the relationship.
Eg update the next shipment date:
Go to Record [First]
Go to Next Record (Exit After Last)
This will create a new inventory record if the product is available and no inventory record exists.
If one exists, it will overwrite the existing value.