how to hook checkout pane defined in another module?
commerce_checkout_form_PAGE is a form ID. Use it inside of HOOK_form_FORM_ID_alter to edit a particular page of the checkout process. Like so:

function MYMODULE_form_commerce_checkout_form_checkout_alter(&amp;$form, &amp;$form_state, $form_id) {
    $form['#prefix'] = '&lt;div id="collapsable-panel"&gt;&lt;p&gt;You have coupon? Add it here.&lt;/p&gt;' + $form['#prefix'];
    $form['#suffix'] = $form['#suffix'] + '&lt;/div&gt;';
}


Alternatively, you can also do a generic alter and check the build info in $form_state:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state) {
  if ($form_state['build_info']['base_form_id'] == 'commerce_checkout_form') {
    // We're in checkout.
    $form[PANE_ID] ... 
  }
}