I think you are looking for the "Create Object" action. It can spawn objects into the scene, from prefabs or objects within the scene. This can take a variable, so you can setup a variable here and assign it various prefab objects. Or you can use the "Get Random Object" action to choose a random object with a specific tag.
"Create action" can also take a position (vector 3) variable. If you need a random location, you can use the "random float" action. Change those floats into variables. This way, you can constrain the area.
I have a tutorial on placing random trees, which may help you out. It shows how to generate random floats and change them to vector 3s, so you can have things generate within a specific area.
. Its pretty close to what you are thinking, except that my objects are not randomly chosen.