How can I attach a line item field to a custom checkout pane?
Got the answer just after posting : so the correct one is 

field_attach_form($entity_type, $entity, &amp;$form, &amp;$form_state, $langcode = NULL, $options = array())

First parameter 

entity_type is the entity type you want to attach, not the one you want to attach to (see why non native english speakers have problems sometimes... Duh !). 

Second parameter 

entity expects an object , not a string ! Don't do

field_attach_form('commerce_line_item', 'my_line_item_type' ... 

but :

$line_item = new stdClass();
$line_item-&gt;type = 'line_item_liberte'; 

field_attach_form('commerce_line_item', $line_item, $pane_form, $form_state, $langcode, array('field_name' =&gt; 'field_paxsup'));


In checkout pane context, load the line item

function MODULE_pane_checkout_form($form, &amp;$form_state, $checkout_pane, $order) { 

  $line_item = commerce_line_item_load($order-&gt;commerce_line_items['und'][0]['line_item_id']);
  field_attach_form('commerce_line_item', $line_item, $pane_form, $form_state, $langcode, array('field_name' =&gt; 'field_paxsup'));
  //...