Well I found a way using the follwing code in the NodeFormatting Event Handler
TreeNodeInfo info = e.Node.Tag
(info.PotentialChildNodes > 0)
e.NodeElement.ExpanderElement.Visibility = ElementVisibility.Visible;
where TreeNodeInfo is a custom class that stores additional info about the node.
This seems to work but now I get some extra space left of the node until I expand it for the first time.