How do you creating a new node in the target structure of existing message mapping

In my Target Structure I have 15 Reference nodes, I would like to add 1 more Reference node, but I am unclear how to do this?

Please provide directions or reference on how to add an additional node to your target structure so that it does not break the current mapping.

the Target Sturcture comes from our customer's xsd. 

Any help  anyone can provide would be greatly appreciated. I am very new to the application and need to add this new node and complete the mapping for my project.

I noticed that the previous 14 Reference must have been added by someone who built the map but I do not know how they did it.

each Reference is