If you use the campaign editor instead of the object editor you can import things to use throughout all the maps in the campaign. You can also use the campaign editor to make custom units, doodads, items, abilities, etc that will work for all maps within the campaign.
Once you make a campaign just right click in the Map Files space under the General tab of the campaign editor and add a map.
Keep in mind you can also edit things individually if you only need to import or edit things in certain maps. You can just use the regular importer or object editor for that. Unfortunately there is no trigger editor that allows you to use triggers for the entire campaign (this would be useful for hero abilities and stuff) Instead you have to open a map, make a trigger, copy the trigger, open another map in the campaign, and paste it. Luckily once you have opened the maps you can use the Window menu to quickly switch between them.
One more useful thing. You can use custom gameplay constants and game interface throughout the campaign. Just make sure you are on the General Tab of the campaign editor then in the Edit menu you can find them at the bottom.
