You cannot roast marshmallows off of the firepit. This is the official answer from the Front Desk (I just called to ask/confirm since I'm here now).
If you want to roast marshmallows, you can go to Ft. Wilderness and do the campire activity which is free to all WDW resort guests. We did this last night and roasted marshmallows and made smores. All of the items needed can be purchased at campire fire. Smores kit is $8.99...forgot how much just for marshmallows. You can always bring your own stuff. They also have movie night and they show a different disney animated classic every night. Last night was Aristocats.