Can a parking pass be added to a membership after the fact or only at the time of purchase ?

Yes you can

Yup

After

It can be added

You have to see the customer relations next to the entrance gate of the park. They can do it for you. Also once a year six flags has a huge sale on memberships. If you renew your pass for the next year during that sale period, you'll get the parking pass for free Enoch is totally worth it.