if i have already purchased my tickets can I add a fast pass to 2 out of 4 tickets

Yes

Yes you can, they are sometimes given deals outside when your in line

Yeap

Yes, you can add on an express option to one or two of the tickets at the entry gate, or inside the park. They inly sell a limited amount of express passes, so purchase it early in the day.

Yes. You can always upgrade, to an extra charge.

Yes, you can go to counter and upgrade 2 tickets to express