DataTable ssp.class.php

I asked this on StackOverflow, but it might be better to ask here since this is the DataTables home forum. I hope this makes sense.

Simply listing the contents of a MySQL table in DataTables, using server side-processing (ssp.class.php) is no problem at all. Works like a charm and I want to stress the fact that this does not concern the function "ShippingMethod" nor how to use regular DataTables in general.

I want to have a column in the DataTable that does NOT have a corresponding column in the MySQL table, and I want to populate this column with computed values, that take other column values from the MySQL table as parameters, how do I do?

MySQL table:
Contains three columns. As you can see, there is NO column here for Shipping Method.

Name | PartnerID | Express
Abe  | 23        | true
Eve  | 47        | false
Joe  | 47        | true

Pseudo-PHP function:
Returns a text-string containing shipping method, like "Plane", "Train" or "Automobile"

function ShippingMethod(PartnerID,Express) {
  return $computedDataTextString
}

HTML DataTable:
As you can see, the column Shipping Method here does not correspond to any column in the MySQL table.

<thead>
  <th>First Name</th>
  <th>Shipping Method</th>
</thead>

Prepping data for ssp.class.php
This is where I run into trouble. How do I get the results of the PHP-function (i.e. Shipping Method) in here? And how would I go about prepping the array, since there is no table column for this data?

$columns = array(
  array( 'db' => 'Name', 'dt' => 0 ), 
  array( 'db' => ???????, 'dt' => 1,