For a little project I needed to list all users, but not with the default user information, but with extra profile_values. Since users were not known by username, but by user id, I wanted a filter for a quick search.

In my module I recreated the form:

function messageboard_form_alter($form_id, &$form) {
if ($form_id == 'user_filter_form') {
unset($form['filters']['status']['status']); // remove status filter
unset($form['filters']['status']['permission']); // remove permission filter

$form['filters']['filter'] = array (
'#type' => 'radios',
'#options' => array (
'role' => role, // keep role filter
'uid' => uid // add my uid filter
)
);
$form['filters']['status']['uid'] = array (
'#type' => 'textfield',
'#size' => 18,
);
}
)

At first I also had the code added to fill the user_overview_filter session array with the values of the added filter, but then I got a SQL error.

At the end I saw no other way then editing the user_filters() function in the user.module like this:
$sql = db_query("SELECT uid FROM {users}");
while($row = db_fetch_array($sql)) {
$options[$row['uid']] = $row['uid'];
}
$filters['uid'] = array('title' => t('number'),
'where' => 'u.uid = %d',
'options' => $options,
);

The options are necessary if you want the text of the search displayed correctly.

I would be nice if there was some kind of hook for editing these filters. If so, please let me know. I really don't like to edit core modules.