Create a read only field. Then setup a Field auto action after the registration trigger. Then set that read only field to [user_id]. Existing users you'd need to update the field manually. Yes, you could use phpmyadmin to update all of them, example as follows.
UPDATE `#__comprofiler` SET `cb_userid` = `id`
Of course make a backup before running any such query. Also change `cb_userid` to the actual name of your field.