UI Support for selecting User Profile Fields

You can completely change the signup form (and control which information should be entered) by modifying theme_signup_user_form() (located in signup.theme).
If you would like to display profile information you can insert something like this (untested and surely not functional!):

if ($user->uid) {
  $form['signup_form_data']['profile']['field_1'] = array(
    '#type' => 'textfield',
    '#title' => t('Field 1 from Profile'),
    '#size' => 40, 
    '#maxlength' => 64,
  );
  ...
}

hth,

 Stefan

Stefan is correct, but to pull the profile fields, you could set a default for the field. For instance, I have mine setup to ask for every person's first name and last name. If they are logged in, then I use the profile fields populate the user's name:

    $form['signup_form_data']['fname'] = array(
      '#type' => 'textfield',
      '#title' => t('First Name'),
      '#size' => 40, '#maxlength' => 64,
      '#required' => true,
    );
    $form['signup_form_data']['lname'] = array(
      '#type' => 'textfield',
      '#title' => t('Last Name'),
      '#size' => 40, '#maxlength' => 64,
      '#required' => true,
    );

    if ($user->uid) {
      $form['signup_form_data']['fname']['#default_value'] = $user_profile['Personal Information']['profile_first_name']['value'];
      $form['signup_form_data']['lname']['#default_value'] = $user_profile['Personal Information']['profile_last_name']['value'];
    }

Sounds like you found out what you needed from other people's replies, calling this a support request and marking it fixed.

The feature you really want is here: #29568: Flexible number and type of fields

Great.