Hi Mark,
by following the instruction reported in
How to write an AddOns for VNL you should be able to write this functionality yourself.
In this case, I made the effort and you can find the plugin attached to this post.
Simply save the file, extract it, and copy the POSCARCartesianExporter folder in the ~/QuantumWise/VNL-ATK-2016/lib/python2.7/site-packages/AddOns/ folder (~/QuantumWise/VNL-ATK-2016 is your installation directory).
Now, in the Builder when you right click on the stash item to export, you will have the "POSCAR Cartesian" option in the file list.
Hope this help.