1) Start a multiplayer game, with the number of players you wish to be in the scenario. Make sure you remember what names all the players are using.
2) Start the game and immediately save it. You should then quit the game.
3) Find the saved game file and change the extension from .ees to .scn and move it into your data\scenarios folder. You can then load it up into the Editor and modify it, adding Triggers or even changing the map. The thing you need to avoid doing is changing the amount of players or their names.
4) When you're done editing, save it and change the file extension back to .ees, and moved it back to your saved games folder. You can now load it back up as a multiplayer saved game- just remember to have the same number of players using the same player names.