I had to pay with card for the initial pledge, but I was wondering if I'd be able to get add-ons with Paypal.
Does anyone here know?
It depends on how they set up the pledge manager. Some do, some don't. CMON's did for Massive Darkness. Also, Monolith's pledge manager for Conan took PayPal as well, so that's a good sign. You'll probably get the best answer by contacting Mythic Games directly, though.