you have possibility to do this :
Example 1
$xcrud->fields('id','field_relation','field_dropdown');
$xcrud->relation('field_dropdown','table_dropdown','id_relation','field_value',array('data_array' => {field_relation}));
Example 2
$xcrud->set_attr('field_dropdown',array('id'=>'dropdown_ajax','data-relation-value'=>'{field_relation}'));
and after this you can use Ajax mode to call id and value and manage dropdown using Ajax using attr you can pass #dropdown_ajax to jQuery or javascript