"Signature" user field not appearing in list of user fields
While it doesn't appear that you can put it in the field list directly and drag and drop it around as the user module only exposes the account and timezone in its hook_fields_extra_fields(), you certainly can move it around "manually" in a hook_form_alter() in a custom module, eg something along these lines:

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  switch ($form_id) {

    case 'user_profile_form':

      drupal_set_message('&lt;pre&gt;' . print_r($form, TRUE) . '&lt;/pre&gt;'); // to see what you have to work with

      $form['signature_settings']['#weight']='8';
      $form['field_my_second_custom_field']['#weight']='9';

      break;

    // other forms go here

  }

}


...which, if you had the following in your user:



...would create this in the edit form:



...eg, we moved the #weight around, which is what the drag and drop anchors do in the UI.

In this example, user.module initially creates the signature settings with a weight of 1, and then the custom fields with 7 and above, so, by changing the weight to 8 for the signature settings and the next field to 9, it puts it between those two fields.  Check out the Drupal Form API and #weight for more information on what this is all about.

If you click show row weights at admin/config/people/accounts/fields you can see this more directly:



ADDITION:  Using Field Groups

If you had it set up like this:



...and wanted to put the signature settings inside the "My Group" field group, you could do something like this:

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  switch ($form_id) {

    case 'user_profile_form':

      $form['signature_settings']['#weight']='6';
      $form['field_my_second_custom_field']['#weight']='7';

      $form['#fieldgroups']['group_my_group']-&gt;children[]='signature_settings';
      $form['#group_children']['signature_settings'] = 'group_my_group';

      break;

    // other forms go here

  }

}


...which would produce this:



YET MORE ADDITIONS:  Tab Groups

If you want to put it in a tab group, eg:



...this will do it:

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  switch ($form_id) {

    case 'user_profile_form':

      $form['signature_settings']['#weight']='6';
      $form['field_my_second_custom_field']['#weight']='7';

      // $form['#fieldgroups']['group_my_group']-&gt;children[]='signature_settings'; // apparently this isn't needed afterall
      $form['#group_children']['signature_settings'] = 'group_my_group';

      break;

    // other forms go here

  }

}


...which produces this: