Populating fields from the User entity during a Commerce checkout
You could attach the fields from the user to the form, but you would need to handle saving the user in the form submit. You could in form alter do something like this

function hook_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'the form id') {
    $form['user_fields'] = array();
    global $user;
    field_attach_form('user', $user, $form['user_fields'], $form_state);
  }
}


You also need to add validate and submit handlers in the form_alter in the validate handler do

 field_attach_validate('user', $user, $form['user_fields'], $form_state);


And in the submit handler do

field_attach_submit('user', $user, $form['user_fields'], $form_state);
user_save($user);