I have written a similar thing with a TreeGrid: http://smo-visittest.appspot.com/ It's only a prototype to show a workflow. Choose the trial xyz from the combobox and D&D the visits to another folder. You could use the "folder" for you variables and don't display some child nodes (hasChildren() always false) and use an empty icon.
The D&D process adds the selected model automatically to the target. In the example I have this problem, if I cancel the visit date dialog. Then I have to manually remove the dropped models from the target store and add it again to the source. I use a DNDListener on source, which stores the selected visits within dragStart() as List and uses the List within dragStop(). There you could make the mapping and revert the List entries from the target.
I'm not sure, where you have to add the DNDListener, because in my example is source and target the same TreeGrid.