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.

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.