Programatically create nodes, taxonomies and relationships via exisiting database
http://timonweb.com/how-programmatically-create-nodes-comments-and-taxonomies-drupal-7 - only thing to note is making sure the taxonomy terms don't exist before you add them, which you can easily do with a function like this (recently wrote this for a project)

function taxonomy_import_add_or_get_term($term, $vocabulary) {
  $tid = taxonomy_get_term_by_name($term, $vocabulary-&gt;title);
  if (count($tid) &gt; 0) {
    $tid = array_shift(array_keys($tid));
  }
  else {
    $term_object = new stdClass();
    $term_object-&gt;name = ucwords(strtolower($term));
    $term_object-&gt;vid = $vocabulary-&gt;vid;
    taxonomy_term_save($term_object);
    $tid = $term_object-&gt;tid;
  }
  return $tid;
}