what exactly do you want to export? Certain items of an App, the whole App with all items, only the content of a field? Should it be a copy only?
To your second question: Create a workspace where all members of the whole organisation are in and a central meeting app. In all Meeting apps in the other Workspaces add a relationship field to this central meeting app. You can now use Podio Advanced Workflows to copy every new created meeting from the other workspaces to the central meeting app or only those who are nation-wide events. If you want to copy only those, add a category field with category "nation wide" and a flow that copies this event when the category is selected. Most easiest way: Use only one central meeting app for all meetings (if that is possible for your structure).