I'm not exactly sure what you were looking for, but I modified your attached application to show one way I might accomplish this.
I just set this up to work with the first list box, but you could extend it to others.
First, I made the listbox so only one value could be selected at a time. Then I created a Macro called AppendtoVar and set it to fire whenever the Qui field had a OnSelect event happen.
The ClearVar button will clear the variable.
I'm sure there are issues to work out and this may not be what you need, but thought it might help you in some way.