Hi,
I looked into this on unity answers (going by the logic of; if you cannot do it in c#, it is very likely (or certain even?) that you cannot do it in playmaker).
So according to unity forums and other support;
There is no way to create prefabs during runtime, only in the editor:
Answer by aldonaletto
Are you using this script in the Editor or at runtime? As far as I know, prefabs are Editor stuff, and you can't create them at runtime. If you want to let the player construct "prefab-like" objects at runtime, simply declare the "prefab" game object inactive to make it disappear of the scene, and use it as a regular prefab in Instantiate (remember to activate the object when instantiated)
Found here:
http://answers.unity3d.com/questions/366273/create-prefab-from-gameobjects-at-runtime.htmlBut luckily i think the above also gives you a solution. If you want to save objects in the game, just create them in the scene and deactivate them. (You can also parent them under a game object which has a 'dont destroy on load' action and then it will be saved in that object's children during loading of levels.)
Then for playmaker you just need to find a way to reference this object in the 'create object' action.
For example, you could use get child and target the object under which you parented the object you wanted to save.
If im not making much sense, or if you generally need a clearer explanation of how i would do it let me know. My main point of this post was really just to point out that it is impossible to save as a prefab during runtime.
Hope this helps.