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

1. open section "Setup fields"

2. click button "Add Field"

3. type  custom meta key in textbox (below the select)

4. fill "Column name"

5. press button "Confirm"

I use "order_number_2" as meta key on the screenshot

I pressed button "Preview" and got  empty column

Now you should create  calculating function

1. Close section "Setup fields"  and open section "Misc Settings"
2. Mark checkbox "Custom PHP code to modify output"
3. Paste following draft function to the textarea
add_filter('woe_fetch_order', function ($row) {
 echo "<pre>";var_dump($row); die(); // remove it
 $row['field_2'] =  $row['field'];
 return $row;

4. We must know keys for already existing fields (for step 5). So press button "Preview" and you will see keys/values

5.  remove line echo "<pre>";var_dump($row); die();

6.  update line  $row['field_2'] =  $row['field'];  
I  used  $row['order_number_2'] =  $row['order_number'];

Finally, I press "Preview" and got 

if button "Preview"  has no effect  - it means there is an error in the code.
So you should  mark checkbox "Enable debug output" too.