Adding Data to a Variable in a Contributed Module
You will not need to 'hack' this module, you can use a legitimate hook to modify the $nvp array, hook_commerce_paypal_api_request_alter

At line 382 of the commerce_paypal_wpp.module, right after the part you mention, the whole array is passed to the commerce_paypal_api_request where the mentioned hook is called early on, to allow modifications from other modules.

Following your example code, your hook implementation would look like this:

function yourmodule_commerce_paypal_api_request_alter(&amp;$nvp, $order, $payment_method) {
  ...
  $fname = 'examplename';
  $lname = 'examplelastname';
  $arr = array($fname, $lname);
  $allthedata = implode('+', $arr);
  $nvp['CUSTOM'] = $allthedata;
  ...
}


Hope this helps!