Navigate to INV : Items -> Cross reference , Enter a unique cross–reference type name and save it.
Click on Assign an old/supplier item and its corresponding inventory item.
2. Indicate whether the cross–reference applies only in the specified organization or to all organizations to which the item is assigned
3. Enter a cross–reference value. This value is the entity you cross–reference to the item, such as its old item number or supplier part number.
Cross reference can also be attached from tools in item master