At first you should add column to export (it will be empty).

1. open section "Setup fields"

2. open section "Products"

3. click button "Add Field"

4. type  custom meta key in textbox (below the selects)

5. fill "Column name"

5. press button "Confirm"

I use "item_tax" as meta key on the screenshot

I moved new field  from right column to left column (list of exported fields),  pressed button  "Preview"  and got  zero values.

Now you should create  calculating function

1. Close section "Setup fields" and open section "Misc Settings" (above it)
2. Mark checkbox "Custom PHP code to modify output"
3. Paste following draft function to the textarea
add_filter('woe_get_order_product_value_{FIELD}', function ($value, $order, $item, $product,$item_meta) {

   $value = "text";
   return $value;
}, 10, 5);

4. replace {FIELD} with your meta key (see 1st phase) .
5. replace $value = "text";  with your code.

Parameters for the hook

$value is string 

$order is WC_Order  object

$item   is WC_Order_Item_Product object

$product is WC_Product  object

$item_meta   is associative array 

In this example, I use meta key "item_tax" and following code to calculate tax per item
$value = $item['line_tax']/$item['qty'];
I pressed button "Preview" and got  necessary values

If button "Preview"  has no effect  - it means there is an error in the code !

So you should  mark checkbox "Enable debug output" to see the errors.