If you feel ok, you could hack the wordpress core files.
I don't know how to do this in terms of plugin or hooks.

In wp-admin/includes/template.php, function get_column_headers(), find and add this:

$_wp_column_headers[$page] = array(

	'cb' => '<input type="checkbox" />',

	'username' => __('Username'),

	'name' => __('Name'),

	'email' => __('E-mail'),

	'role' => __('Role'),

	'posts' => __('Posts'),

	'custom' => 'Custom' /* Add here your custom field Id and Caption */

);

break;

Then find the function user_row() in the same file and add this in the switch-case after the posts case:

case 'posts':

	$attributes = 'class="posts column-posts num"' . $style;

	$r .= "<td $attributes>";

	if ( $numposts > 0 ) {

		$r .= "<a href='edit.php?author=$user_object->ID' title='" . __( 'View posts by this author' ) . "' class='edit'>";

		$r .= $numposts;

		$r .= '</a>';

	} else {

		$r .= 0;

	}

	$r .= "</td>";

	break;

/*

	Your field

*/

case 'custom':

	$r .="<td $attributes>Your Values</td>"; /* Here you should retrieve the custom field value associated to the $user_object->ID */

break;

For retrieving the custom field value for each user you can make a raw MySQL query in the custom case
You could use the variable $user_object->ID accessible in the switch-case.