Hi,
Typically, you want to use the "Don't destroy on level load" feature of unity ( action "DontDestroyOnLoad" in playmaker).
this will make your object survive level loading.
the downside of this is during development it becomes difficult to edit a particular level since you won't have the prefabs you have as you start the game for real.
enters "Singleton", you can ensure with various technics that if an object exists already, then you destroy the duplicate. So for a given prefab that should be there at all time throughout your game, make it as a singleton, add it to all your levels and then during editing, you can play a particular level as is, and during the game, it will manage itself to maintain only one of this prefab at all time.
http://hutonggames.com/playmakerforum/index.php?topic=6481.msg31681#msg31681 search the forum for "singleton" and you'll get plenty of experience and technics.
bye,
Jean