passing a value from exposed filter to the views arguments using form alter

hi -
i made a form_alter function to add a new select box that i didn't defined in views and i want to use its value after a small processing and pass it as the third argument of the view - here is the code i wrote - but somehow it works only the second time i click the exposed submit button. any suggestions?

<?php
function plimus_form_views_exposed_form_alter(&$form, &$form_state) {
  if ($form['#id'] == 'views-exposed-form-lounge-offers-page') {
    $categories = taxonomy_get_tree(4, 0, -1, 1);
    $tax[0] = 'all';
    foreach ($categories as $cat) {
      $tax[$cat->tid] = $cat->name;
    }
    $form['category'] = array(
      '#type' => 'select',
      '#title' => t('Category'),
      '#options' => $tax,
    );
    $category_tid = $form_state['view']->exposed_input['category'];
    $children = taxonomy_get_tree(4, $category_tid);
    $terms = array();
    foreach ($children as $term) {
      $terms[] = $term->tid;
    }
    $path = explode('/',$form['#action'],5);
    $path = array_slice($path,0,4);
    $form['#action'] = implode('/', $path) .'/all/'.implode('+', $terms);
  }
}

what i try to do here is to add an exposed filter of only the parent taxonomy term of selected nodes.
i know there is an issue that tries to deal with it by making a patch to the views module here - http://drupal.org/node/271833 , but my question tries to be more generic - is it possible and if so how to add a custom form element on hook_form_alter so i could pass a processed value to a predefined views argument?

thanks