I have found the easiest way to do this is to loop through the list then assign it to the ArrayList.
1) Get the proxy from the game object:
PlayMakerArrayListProxy goProxy = go.GetComponents<PlayMakerArrayListProxy>()
2) Then loop through the List with a foreach and add the info:
foreach (var item in list) {
goProxy._arraylist.Add(item);
}
The key here is the _arraylist. It is what is exposed to access the arraylist data itself.
Hope this helps,
Z