Shopify stores the HS Tariff Code (or Harmonized System Code) data as a product variant metafield.

In EZ Exporter's Order Data Settings or Product Data Settings, scroll down to the "Metafields" section and use the key harmonized_system_code in a Product Variant Metafield.

Update Feb. 21, 2019: This field is also now available via the InventoryItem resource (plus additional related fields such as the Country of Origin). We recommend that you use this new field as Shopify will deprecate this Metafield key in the future. More info here.