We are not the developers of "MySQL Field" so I can't advise you on its usage, but our Professional subscriber plugin CB Query Field can certainly do this. The alternative is to use CBSubs SQL Actions integration to insert the expiration date into a users field. An example query to do this with our CB Query Field plugin as follows, which may help with configuration of your "MySQL Field" plugin.
Code:
SELECT `expiry_date` FROM `#__cbsubs_subscriptions` WHERE `user_id` = '[user_id]' AND `id` = PLAN_ID_HERE