Adding CCK field to solr index
Adding Custom Fields to Apache Solr Search Results appears to have the instructions.


  To do this, we will use the two hooks used in the last example: hook_apachesolr_update_index() and hook_apachesolr_modify_query().  To start, we need to add the data to the Solr index.


Modified the code to work with a field_keywords_field (note: field_keywords_field[0]['value'] is what I'm using, you may need to debug this if your keyword field is multi-value or something)

/**
* Implementation of hook_apachesolr_update_index()
*/
function mymodule_apachesolr_update_index(&amp;$document, $node) {
  // Index field_keywords_field as a separate field
  if (isset($node-&gt;field_keywords_field) &amp;&amp; !empty($node-&gt;field_keywords_field)) {
    // addField($key, $value, $boost), change $boost to give this field a higher score
    $document-&gt;addField('sm_field_keywords_field', $node-&gt;field_keywords_field[0]['value'], '2.0');
  }
}

function mymodule_apachesolr_modify_query(&amp;$query, &amp;$params, $caller) {
  $params['fl'] .= ',sm_field_keywords_field';
}