You can export again starting from 251.
The 250 limit is there to avoid timeouts and memory problems. Still, if you want to modify this, edit s2member-pro\includes\classes\exports-in.inc.php and look for the line:
[hilite pre_code]
if (is_array ($_users = $wpdb->get_results ("SELECT `" . $wpdb->users . "`.`ID` FROM `" . $wpdb->users . "`, `" . $wpdb->usermeta . "` WHERE `" . $wpdb->users . "`.`ID` = `" . $wpdb->usermeta . "`.`user_id` AND `" . $wpdb->usermeta . "`.`meta_key` = '" . esc_sql ($wpdb->prefix . "capabilities") . "' LIMIT " . $sql_s . ", 250")))
[/hilite]
Change the [hilite mono]250[/hilite] at the end to what you want.
I hope that helps. :)