How do we preprocess CSV columns when using CiviCRM builtin import functionality?
One possibility could be importing the data as it is (maybe using a temporary field for that column) and use 
hook_civicrm_post() to manipulate the contribution just after creation:

function example_civicrm_post($op, $objectName, $objectId, &amp;$objectRef) {

  if ($objectName == 'Contribution' &amp;&amp; $op == 'create') {

    if (!empty($objectRef-&gt;temporary_field)) {

      // Preprocess field. FIXME: do some validation first.
      $objectRef-&gt;real_field = do_awesome_stuff($objectRef-&gt;temporary_field);

      // Clear temporary field.
      $objectRef-&gt;temporary_field = '';
    }
  }
}