Adding column to invoices

i am just start use this software. i want to know to i add other column in stock inventory item, sales and purchase invoice. for Example my work is food item relates and i have one item ABC so i need how much item ABC boxes and what is the weight of box. like i have 150 boxes and weight is 1545 kg

Have you read the three parts of the Inventory Management Guides? Those link to other more specific Guides.

in a three parts of the inventory management guides have not my answer. i just asking you simple can i add one more column in purchase or sales and inventory. you have three column. 1) Qty. 2) Unit Price 3) Amount.
i want this 1) Box (whether Box name not) 2) Kg (like Qty) 3) Price 4) Amount

read about custom fields in the guides section.