Where does Drupal 7 store custom user account fields?
Assuming you're using the SQL storage backend they're stored in the field_data_field_FIELDNAME and field_revision_field_FIELDNAME tables. The field system in Drupal 7 is generic across all entities so field data for nodes/users/all entity types is stored in the same tables.

Rather than input directly into those tables, though, I'd suggest using the Drupal API to load, alter and then save each user instead. The code might look something like this:

// Grab all the uids in the users table
$uids = db_query('SELECT uid FROM {users}')-&gt;fetchCol();

// Load the users
$users = user_load_multiple($uids);

// Loop through and edit each user
foreach ($users as $user) {
  // Load the extra data from whatever source you're using
  $extra_data = my_function_to_get_extra_data_by_user_id($user-&gt;uid);

  // Update the fields
  $user-&gt;field_FIELDNAME1[$user-&gt;language][0]['value'] = $extr_data['FIELDNAME1'];
  $user-&gt;field_FIELDNAME2[$user-&gt;language][0]['value'] = $extr_data['FIELDNAME2'];
  // etc...

  // Save the user object
  user_save($user);
}


You'll probably want to change this code to process the users in batches as it's likely to take a long time to run.