Or you could keep your bar the same, and buy an 1 1/2 galvanized pipe. It's sturdy as shit, and it fits olympic plates damn near to perfect.
why would you want to put oly plates on a pullup bar? But you're right, just buy a 2" galvanized pipe and find a way to attach it to your ceiling or exising pullup bar. Hang it, or buy some more pipe fittings to screw it to a truss, or something.