Does CiviRules work with smart groups?
Contacts don't really get 'added' to a smart group - it's more that when the smart group is used, a search is run to find who currently meets those criteria (glossing over some complex rebuild and caching details...)  So there is no specific event for CiviRules to react to in a post trigger.

You might be able to base something on the 'Daily trigger for group members' cron trigger and maybe modify it for a more frequent run.  See https://docs.civicrm.org/civirules/en/latest/trigger/