Have a membership from fiesta Texas San Antonio and my question is it possible that I would get access to this membership.??

If you have a gold membership or above it's good at all six flags parks.

Yes I have the same membership from Fiesta San Antonio and I Just used it there and got in... and and the parking too. They both are great places..

As a heads up this week change by the 2023 season which starts the second weekend in January. So please check the website regarding these changes.

Yes!

And Frontier City in Oklahoma City :)

Yes, it should work there especially if you have a higher tier membership

Regular season pass covers parking at all the locations but you need to add at least the gold membership to get into all parks I believe.

Yes. I have one and it works.

Six flags related you're good to go

I think so, yes