How do I pre-populate the login/sign up form?
I have already extended the standard Drupal login form


Sounds like you're already using hook_form_alter(), so all you have to do is add the #default_value attribute to your form array. 

Ex:

$form['my_field_name'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('something'),
    '#default_value' =&gt; t('Hello World'),
);




OR Use the Profile2 module suggested by @Kevin


  Profile2 allows for creating multiple profile types, which may be
  assigned to roles via permissions (e.g. a general profile + a customer
  profile)