At first you have to start a game with more than 8 players. How you can start a game with more players is described in the
FAQ Q7. Now you can put settlers on the map (of course if you don't like the map load another one or modify it), finnally you have to select the option Full Game from the scenario editor nation tab. When you done everything save the scenario (that should as you said in your initial post only contain a premade map). Than go to the SP game start screen and start your map/scenario and have fun.
-Martin
