Conditional mapping with Feeds
The easiest is to just put a formula into a spreadsheet and alter your source csv.

Else you could write your own feeds tamper plugin.

Create an empty file called 'stock.inc' and place it into the feeds_tamper/plugins directory. Add this to it:

&lt;?php
$plugin = array(
  'form' =&gt; 'feeds_tamper_stock_form',
  'callback' =&gt; 'feeds_tamper_stock_callback',
  'name' =&gt; 'Calculate stock',
  'multi' =&gt; 'direct',
  'category' =&gt; 'Other',
);

function feeds_tamper_stock_form($importer, $element_key, $settings) {
  return array(
    'help' =&gt; array(
      '#markup' =&gt; t('Sets the stock status'),
    ),
  );
}

function feeds_tamper_stock_callback($result, $item_key, $element_key, &amp;$field, $settings, $source) {
  $field = $result-&gt;items[$item_key] &gt; 0 ? 'In Stock' : 'Out of Stock' ;
}


Clear your cache.

I have not tested this but I believe it will get you close.