Add Column to Administration Shopper List

[ZoBabe]

Joomla 2.5.27
Virtuemart 2.6.8
PHP 5.4.30

My client has a custom field for a reference number he assigns manually, which we need to have show up in the administration section shopper list next to the user name. No problem adding the extra column and header, but cannot figure out how to to get it to grab the custom field value from the database. So, the column is entirely blank, even when the shopper field does contain information.
  Any help would be greatly appreciated!