As far as I know, they do not offer refills (except for when they're running the AP popcorn offer, but I don't think there is one currently being offered). The best you can do is buy the regular popcorn, in the cardboard box, and then pour it into your round container. Unfortunately, that won't help with the waste, but at least then you don't have to worry about spilling the popcorn.
Same for drinks; they don't sell refills and they (normally) will not pour a drink directly into a previously purchased cup. You'll have to buy the drink and then transfer it yourself. They've said it's because health code requirements do not allow them to take previously purchased items behind the counter.